Trailing arms and bags on front?

What I can clearly see after completing my front axle flip is that the curvature and axle placement on the front leaf springs of my 68 108 is that.....

1) the bottom of the leaf's arc is ahead of the u-bolt saddle and a bit lower at about
2.5"
from the ground while the rims are 4"...... This means that the leafs can bottom on
2.5"speed bumps before the tires get there...
2) Because the axle is behind the bottom of the arc, several more degrees of wedge
shims
were necessary to get enough caster and this resulted in the axle being an inch
above
the leaf springs (an extra inch of lowering)
3) Rolling the axle 12 degrees back on the springs makes that "extra inch" necessary in
order for the tie rod to pass freely over the top of the leafs
4) The only way to fix THIS ground clearance issue is to take that inch back and
switch
to 15" rims, which would put the ground clearance at exactly rim height...

At this point, I'm pretty much committed to the setup I have, but that doesn't stop me from considering how I coulda/mighta done it differently...... So consider this....
What if (?)....

Picture the front leaf springs being replaced by a pair of ladder bar/swing arms that fasten into the front spring hangers and run up and down in channels at the rear, instead of shackles.......
Axle flip would be built into the top of each bar and air bags would float it from right behind the tie rod....
These bars would probably need to be round so that the flip saddles could rotate on them for suspension purposes (like when one wheel is up and the other drops)

Some will say that just swapping an s10 frame stub is the way to go, but I believe that early vans lose their soul when they lose their straight axle...

AzDon wrote:What I can clearly see after completing my front axle flip is that the curvature and axle placement on the front leaf springs of my 68 108 is that.....

1) the bottom of the leaf's arc is ahead of the u-bolt saddle and a bit lower at about
    2.5"
   from the ground while the rims are 4"...... This means that the leafs can bottom on
    2.5"speed bumps before the tires get there...
2) Because the axle is behind the bottom of the arc, several more degrees of wedge
    shims
    were necessary to get enough caster and this resulted in the axle being an inch
     above  
     the leaf springs (an extra inch of lowering)
3) Rolling the axle 12 degrees back on the springs makes that "extra inch" necessary in
    order for the tie rod to pass freely over the top of the leafs
4)  The only way to fix THIS ground clearance issue is to take that inch back and
     switch
     to 15" rims, which would put the ground clearance at exactly rim height...

At this point, I'm pretty much committed to the setup I have, but that doesn't stop me from considering how I coulda/mighta done it differently...... So consider this....
What if (?)....

Picture the front leaf springs being replaced by a pair of  ladder bar/swing arms that fasten into the front spring hangers and run up and down in  channels at the rear, instead of shackles.......
Axle flip would be built into the top of each bar and air bags would float it from right behind the tie rod....
These bars would probably need to be round so that the flip saddles could rotate on them for suspension purposes (like when one wheel is up and the other drops)

Some will say that just swapping an s10 frame stub is the way to go, but I believe that early vans lose their soul when they lose their straight axle...

Might think about making a short set of truck arms similar to what the rear suspension on some early 60's C-10. Basically a short I-beam. These arms could bolt directly to the spring perch, as they do on the rear axle of the aforementioned trucks and could incorporate an airbag mount, off the rear. The front of the arms would use HD rod-ends. You'd need a panhard as well, to locate laterally, unless you moved the front locations inward and triangulated OR put a diagonal link from the front of one truck arm to the rear of the other. I say all this without having crawled under mine to see how it might all work, so its entirely theoretical at this point.

The biggest issue with trying to replace leaf springs with something more rigid is.....
The leaf springs are designed to hold the axle (more or less) in place against front2rear, side2side, and twisting forces..... The axle is a solid beam that resists being twisted by twisting on the leafs instead.... If one wheel is up, both leafs twist a bit and pull closer together and the caster is averaged between the two sides because the axle won't twist.....
Solid bars would likely not allow any independent suspension movement between right and left without binding.....
I've got to give the whole concept a bit more thought...

Yes, you are correct.  But as I said, the truck arms also bolt rigidly to the axle and were used for decades in NASCAR.  They are designed to twist a bit.  The I beam is essentially two C channels welded back-to-back with short 1" or so welds spaced a few inches apart on the top and bottom of the I.  Play with the wall thickness to get the desired amount of twist as expressed in degrees of twist VS torque applied..  Mount on a tangent to each other (as they did on the C10 and NASCAR race cars) and no other triangulation is needed.  
And with high-misalignment Heim joints at the front, (or even rubber or poly bushings) there would be some freedom of movement before twist would onset. I really believe this could work quite well.  I'm talking myself into using this approach at both ends of my van!

I had never considered that truck arms could twist radially and they are quite long, so any radial twist of a rear axle during body roll (one wheel up, one down) would be minimal...
Truck arms also use only two u-bolts, that go through the truck arms dead center, and while they do point inward, they still use a panhard bar to keep the axle underneath on turns....
A four-link with eight fat bushings or heim ends solves all binding issues, but must use a panhard bar, which doesn't exactly keep the wheels centered under the vehicle, as the end fastened to the axle travels in an arc.....A four bar DOES maintain caster through it's travel arc though....
In some ways, side2side bind could be a good thing, as it would resist side2side body roll and minimize any need for a torsional stabilizer bar, but it might also twist the van into a pretzel as one wheel hangs turning into a driveway. The chassis on these vans is pretty flimsy and mine may be especially vulnerable because I cut the center box crossmember out (the one the trans originally hung from) and my new trans crossmember doesn't really re-create that structure.....