Tranny slip

Transmission slips real bad, is there any chance that it might be the converter instead of the trans. It is a 727 and slant six engine.

Recent purchase, drove the truck home ( 1973 ) but was slow going. Forget about getting any RPMs

Not sure how to diagnose, just assuming the trans is bad.

Not certain what is going on as its parked in the dirt right now but the guy had something under the truck cause of the tranny leaks. Im assuming that it has leaked for a while and he let it go too long between fill-ups but that is just a guess.