Rear main seal

Anyone know if it's possible to replace the rear main seal with the engine, or engine/trans in place on a 65 170 w/3 speed?

You should be able to. You'll have to suspend the engine from above so you can remove the oil pan. I'm pretty sure you'll have to pull the trans to get the crank to drop down but I'm not sure. Then you'll have to loosen all the main caps and drop the crank/rods/pistons down about 1/32". Pull the rear cap off and pull out the old seal and use a "sneaky pete" to pull the new one around the upper groove above the crank. The seal should be clocked so the ends are not at the mating surface of the cap. If you want to go with a newer type lip seal you'll have to drive the pin out of the cap's groove that holds the rope seal from spinning. And be sure and face the lip in the right direction. It's important to seal the ends with a little rtv. I'll review my book later if you want

Seth, thanks for the info. I was thinking about not being able to support the engine from below once the oil pan is off after I asked the question. Im pretty sure I need to replace the clutch because of all of the oil leaking onto it. It is slipping pretty bad. I was starting to think maybe I should try to do the seal with trans in place, put the pan back on, put a Jack stand under it, then remove the trans to do the clutch, but you made a good point about not being able to lower the crank with it still attached to the trans. I can't think of any way to support the engine from above since it's a pickup. I definitely want to go with a new rubber seal instead of a rope seal. Is there a pin in the block under the seal as well, or just in the cap? (Hopefully just the cap)

It's just in the cap. If you have access to an engine hoist, you can hang it from above, then pull the trans and oil pan. One of the problems is you can't get the oil pan off with the cross member in w/o either lifting the engine up or removing the crossmember b/c of the oil pump and pick up. Tice and I ran into this problem fixing his blown piston. Iirc, we were able to lift the engine enough with the hoist to *just get the oil pan out without removing the crossmember. I've seen pictures on here where Don in San Diego just used a steel tube or angle across the doghouse on elevated jack stands to suspend the engine with chain. I assume he jacked it up a bit first off the mounts with a floor jack then hung it off the steel, but that was in a van... You may be able to clean an oil soaked clutch with brakleen or the like but I don't know. I'm sure someone on here has done this job on a 65-67 170 van, hopefully they will chime in. On the 61-64's you can drop the pan b/c there is no crossmember just engine hangers on each side.

Btw, while you have the trans off inspect the camshaft block plug, that one can leak oil as well.

Seth, thanks for the great info. Maybe it is the camshaft block plug that is leaking. I'm just assuming it's the rear main Street the oil is running out of the bottom of the bell housing

A couple of jackstands,,,,a pipe across them,,,,and some chains down to motor mounts, lets the engine hang so you can get the crossmember and  pan off,,,,,rear main might be this type that just slides out around crank,,,,

I'm so glad I decided to ask questions here first. I would have found all of this stuff out the hard way. Thanks again

No sense in you finding out the hard way. That's what we are here for.

On my 1964 170 cid there is a grafhite rope seal on the rear main cap half. The other half of coarse is around the crankshaft in the block. There is special tool that looks like a chineese finger hand cuff if you ever seen one of those. Yes the job can be done without removing the engine or trans. Remove oil pan and spark plugs and loosen belts so you can turn the crank by hand. My 64 had cantilever arms as motor mounts and not a cross member under the engine. I did this repair in the 1970's when I was a young man. It can be done sucessfully.