I have a 78 F150 2wd with a 400. I am changing the rear main seal. I have the motor on 1" blocks so I had enough room to remove the oil pan. It is a 2 piece seal, so shouldn't I be able to access it inside the crank without removing the transmission or motor? Also, do they make a sleeve for this? I have had this trick for 25 years, I just decided to put on headers, high volume oil pump and rear main seal to stop the leak i have always had.
20 years ago i put a Holly Pro-injection fuel system on it and wow does it go!!!

 

I hate to say it but the best way (IMO) is to pull the engine. I say this because it is just easier to do this on the engine stand with the engine upside down.
That said, you can do it in the truck by pulling the pan, rear main journal and loosening all the main bolts slightly you then pry the crankshaft down a slight amount and then you CAREFULLY push the half of the seal that is in the block out and then the new seal half back in; then reassemble in reverse order.

 

Hard to get the upper seal in proper. Rope seal. .. a few tricks. Done before. Involved a bit of packing and trimming. Rubber seal. ... easier. But no fun engine in.

 

I always bought 2 seals, you have to use 2 uppers to get one not to leak. pull the first one in with a wire, past where it is flush. push the second one up to the end of the first.
trim both off a bit passed even.
place the bottom one on, and use form a gasket between
Like John said it is a rough job
It will still leak

best to pull the motor, and on a stand