727 been sitting.

I have a 1972 swinger with 727. Car had a 340 which is no longer usable, putting in a 360. I have never driven car so I have no idea of condition of transmission. The guy I bought it from never drove it either. The car has been sitting at least 4 years. I have two 360's, one I'm rebuilding for when the car is done and another that runs but has 120k on it. My question is what to do to trans? I thought about putting the running 360 in and testing car out until I can start rebuild, I am working on a 49 Chevy project now. Should I do something to trans before testing? Or am I better off pulling trans and rebuilding it? I also don't know mileage on trans because car was originally slant 6. Just wondering if running trans could hurt it after sitting, or knowing condition. Thanks.