1985 Toyota Pickup Oil leaking between bell housing and blo

I have small amount of oil leaking from where the bell housing mates to the block. Lately my clutch has gotten really slippery also, has trouble grabbing gears and often slips even after a completed shift. I assume oil is leaking onto my clutch plate, but I don't know what from. Is there a main seal on the back end of the block that could be shot? I don't know how obvious the leak point would be; I plan on dropping the tranny to look, but obviously need to fix the leak before getting a new clutch plate.