The rear main seal can leak because of too much crankcase pressure. Make sure all the plumbing to and from the flame trap is clear- including the small hose and nipple leading to the intake from the flame trap holder. Also suspect the black oil seperator box mounted to the base of the the block under the intake manifold. I recently had a rear main seal blow out because of restrictions in that box. Remove it and use drillbits and solvent to clean the openings and inside.

If you must remove the transmission it can be done using jackstands. I never measured how far up mine put the car but 20 inches sounds reasonable. Having the car 10 inches off of the floor is as dangerous as 20 inches. To relieve my anxiety I always try to shake/push my cars once they are on the jackstands to see that they are solidly placed. You are wise to be concerned about a car falling on someone.

I have a transmission jack I bought from Harbor Freight for about $60. Makes the job much easier. I have seen people make adapters for their floor jacks and some have worked OK- others not so well. The transmission jack helps a lot in reinstalling the transmission.

Randy