SB 727 troubleshoot

Hi, hoping a trans guru out there can diagnose this problem.
Put this 727 trans on 318 into dart just for ease of installation.(by myself)
Trans came with 318 engine I bought. Knew nothing about its condition.
All I did was change filter/cleaned pan didn't smell bad or burnt or metal shavings visible.

Car shifts but only after being in neutral a while. Seems sluggish but engine runs great.
When I first put trans fluid in started up in neutral, put in drive nothing. well a few quarts overfilled then car starts to move and shift. Let warm up shut down. Next day gallon of trans fluid under car I believe from bottom of filler tube???? Not pan. No leaks until after shut down when hot.

Torque converter?

Thanks for your time.