You're going to want to replace all those external seals (like the crank main seals and the output shaft seals and the like) with ones actually from Toyota. Toyota found out that the original ones turn hard (as you mentioned) but then start cutting grooves in the shafts as they get old. Something odd about the composition. If you put an aftermarket seal in there, the new seal rides in the groove the old one cut and leaks again after not terribly much longer - it's specced to the original seal that was in there. Toyota released updated seals in the 90s that move the lip forward/out a bit and allows the lip to ride on a 'fresh' surface instead of in the smaller diameter groove so it can actually, uh, seal. None of the aftermarket seal kit makers seem to have twigged to this for 70s and 80s Toyotas.
Also, remember to grease the lip of the seal prior to installation. Lots of people still forget that one.