I need some troubleshooting with what i think is a shifting problem., here's my specs and problem

72 Cutlass S with 455
Rebuilt 350T transmission with shift kit
370 (I think) gears, eaton posi
10 bolt rear end, I think original
Hurst 3 speed floor slap shifter
Trans fluid level is good
New u joints


I have a very loud "clunk" that even shakes the car... it feels like someone ran into me. it only happens in drive, at low speeds, when it upshifts into next gear. if i take off in 1st or 2nd and then shift into drive its fine.

Thanks for your feedback!

 

I had a TH400 behind my 461 that did the same thing, at times, and it is very annoying. The trans was fine and had a shift kit in it and thats just the way it acted at times, at very low speed like a few miles an hour going over a speed bump for example.