tranny trouble

[oldschooler]

Hi everyone,
I am having trouble with a 1972 350 turbo. I have no park. I just bought this truck and the guy I bought it from said the tranny needed rebuilt, It was slipping so bad it wouldnt even move in reverse, and barely moved in drive, So I made my offer and he bit. When I picked the truck up I added almost a gallon of tranny fluid and drove the truck home ( It costs to be mechanically un-inclined ). The tranny goes through all gears fine with no slippage. I just dont have park, a Couple of times it actually surprised me and went into park. My question is: Can I investigate this problem by dropping pan or is it more internal than I want to go? Thanks for any help, Oldschooler