Rebuilding a 300 L6

I hope when you say adjust the clearances on the rockers you simply mean adjusting them as hydraulics and not solids?? LOL,, I took and old valve cover and used a hole saw over each one for adjusting them while its running,, the BEST way to do them.
As far as putting in a big six in an early, the biggest problem to over come is having to use the late taller doghouse with a hump in the rear. The rest is no big deal, depending on how you attack it, which I would do this way.
I am not a big fan of the early trans mounts, even the little sixes take em out, so prefer a stronger mount. Here are the ways to handle that issue

1. Big six tranny, early donut tail on it. Problem is you are STUCK with where the engine goes, both in height and forward/rearward as the tranny mount is the fulcrum. You are also still at the whim of using the weaker donut mount, now with a bigger motor???????? not really very good. The late doghouse and hump taking care of the other issues of course.
2. Use a GENII stick tranny with an under the transmission mount. It is a top loader also, a REAL transmission mount underneath, solid output yoke and short tail. This allows you to mount the package where YOU want to put it, drive line angles, radiator and tie rod clearances, etc,, bell housing clearance, and so on. Actually with the late doghouse, and this combo of a new front member as well as now the tranny cross member is really the best way.
3. DONUT MOUNT TO LATE TRANNY USING ADAPTER MOUNT. Even though you are using a late tranny with the adapter you are still back exactly like #1, stuck in the position that the adapter puts you at??? as well as back to square one with a WEAKER donut mount as a final.
In my opinion, if it took .060 to get it where it is, I wouldn't even go any further with it. Problem is, when you first posted about the jugs stuck I already figured the block was trash in the first place, one look at the picture and your comment, I just shrugged my head, and said,, here we go again,,
Not trying to be offensive, just honest,,,,
vic

I wish I had another valve cover so that I could prevent oil from splashing all over my intake, but yeah... now that the engine has run and warmed up, I can verify top end oiling and adjust the rocker pivot nuts properly... twisting the pushrods until they have the right tension, now that the lifters have been oiled... I make such a mess adjusting my valves, as I too prefer to do it with the engine running... instant auditory feedback!

So a tranny from a Mid-Ford? I don't think I have access to a later doghouse and floor hump just yet (nor the tranny). I like your option two, if I can find a mid, or maybe a similar vintage F150?

No offense taken on the engine... DemonDog gave it to me in case it could be saved... through the course of going through it, and realizing some of the challenges / changes big sixes went through shortly after they were introduced... it's still a can-o-worms and was pretty close to becoming a paperweight, but not a lost cause. Removing those last two pistons was a destructive process, and rusting was evident. I could afford a free engine at the time.

I initially tried to find a rusty F150 with a 300 for cheap locally, but there was nothing except with 302s and 351s.

Right now, the block is on the stand with the bores and machined surfaces coated in clean motor oil, as are the crankshaft and camshaft. The oil pump is now installed in the van, the fuel pump is rotten... but at least the block is waiting patiently to be honed, crank to be turned .010, shop to order the pistons (unless they match one of the eBay deals), and for me to build it up as much as I can. Still will have to get head work done...