Help troubleshoot 904? Slipping at higher RPMs

I bought a rebuilt 904 Trans from someone February. They had bought it from someone else a while before that for a project that never happened. Not sure how long the Trans sat after rebuild before I got it and I just got it installed a few weeks ago.

Trans has reverse pattern manual valve body, 5.0 tci shift lever, Low planetaries, Mopar steel deep pan.

I had a new custom converter built by Msaine that should stall around 2800 to 3000 Put in. Rear end is 8 3/4 Suregrip with 3.91 gears. Engine is 69 340 that should be making close to 400hp as built.

The problem I am having is it slips at higher RPMs in first and second. Super firm shifts but at about 3500 to 4000 RPM the trans slips or just drops out. Soon as that happens I can shift it to second, Barks the tires and goes strong to about 4500 and same thing. It does not slip in 3rd (At least that I have seen yet but I have not been out really getting on it and have driven the car very little). No slipping at all at lower RPMs.

Not sure where to start troubleshooting with this. Any suggestions, tips or opinions would be welcome.