I feel like talkin. Hope you feel like readin.
First when you have an older motor lets see what you have as far as the hard parts are concerned.
Get the thing warmed up and take her out some. Then pull the oil filler cap and watch it. Bring it up to full load. There should be very little smoke and vapor coming out of there. If there is a lot coming out of there you have just determined that all the gaskets and seals won't help. That is good to know. There are cheap PCV testers at auto parts stores to give you a further clue. Any pressure from the combustion chamber that gets into the crancase is not good. Actually it's bad. A stuck or broken ring will let the fire down past the piston. There are a whole bunch of scenarios from there on. (none good) IF OK then---->
See if your compression is even. Take a piece of chalk (motor still warm) and mark your crankshaft pulley where the marks from the factory probably are. Then chalk 180degrees from that. Turn your motor over (with a ratchet wrench if necessary). Don't break the bolt off. As you turn the motor over as each mark approaches they should feel the same. Turn the motor at least twice on each mark. If you run into one that turns past the mark much more easily than the other ones you have a compression problem. If you have a compression problem don't bother with new gaskets etc!
If for some reason the main bearings are a bit loose the rear main seal will soon leak again. You can check this by putting the crankshaft 90deg off one of the marks and then moving the flywheel up and down. A dial indicater is nice to have and if you have one you can figure out how to use it. The flywheel should not move vertically more than .0025" That is 2 and1/2 thousands. The bearing wear in that type motor is mostly vertical. If you can see any difference pushing the flywheel from side to side as opposed to up and down you have a hard problem.
You can use an electric lift pump. About $40. If you need some more talk let me know.
You can fool around a long time with gaskets and seals without fixing very much if a hard part is worn out. (Rings,pistons,bearings,valves, sleeves etc.)