Hello Hutch, Just installed a race built 400turbo w/ manual reverse valve body and an 8" Art Carr NL convertor. Trans is very slow to go into reverse or trans break engagement. Is this a servo piston leaking? Convertor? This is a new trans w/ no runs on it.
It is possible that the rear servo piston is leaking ,likely at the cover to case gasket or there is a problem with the direct clutch.
I don't know the make of your transbrake valve body but some designs really do not have a very good reverse circuit and it is advised to put the shifter in neutral and hold on the transbrake button to back up.
The converter won't be the cause of this problem. First verify fluid level and dipstick accuracy. The trans with a cooler and 8" converter should have taken approx 8-9 quarts. Check the linkage in 1-2-3 carefully.
Let me know what you find , Hutch