powerglide won't go forward

I'm working on a 67 buick special wagon,it sat for about 10 years I've changed oil and filters on motor and tranny,however tranny seems to have a bad seal cause leaks oil when sitting awhile,but was working fine,now that I was ready to test drive after sittng for about another 8 months,(fluid was low but I added)it won't go forward,reverse is fine!!! so I jacked the rearend on stands,if I put it in low and run at a high RPM the wheels start moving forward little by little then I can put it in drive and it kicks in but will only stay in drive at a somewhat high RPM,if I slow it down with the brakes to a low RPM it seems to kick out of gear:confused: then I have to start all over again in low gear,does anyone have any idea whats going on??? when it comes to transmissions I am clueless, I do have another powerglide off a 70 GM (chev.) does anyone know if it's the same tranny?? :confused: