Just rebuilt my trans (t-350), replaced the pump and intermediate drum. Using the same valve body/shift kit as before. Now running a Hughes GM-30 converter (advertised stall is 3000).
Problem is that it won't shift (Manual or Auto) below 4500 rpm, and if I shift into 3rd at 4500 and let of it acts like it is in neutral until the RPM's come back up. Pulled the pan, recleaned the valve body (looks good), checked the govenor (looks good), Fluid is full and clean.

Any Help as I am "Brain Dead" now.

did you check vacumn or that the kickdown linkage isnt right.

It reads like a problem with the oil pump, not pumping enough until 4500 rpm. On the side of the trans is a pipe plug, there are several, but the closet one to the bellhousing is pump outlet. You might hook up a 0-min200 PSI gauge to it to see the pressure made.

Go to your local trans parts shop and get a ATG manual for the THM350, it will tell you everything you need to know.