1979 Dodge B300 5.9L V8 360 2bbl, A727 trans.

Ok so this is a fresh rebuild of a stock 727.
I have less then 10 miles on it and it's slipping in Drive 1st and slipping in Second 1st. Reverse and manual 1st are fine.
I checked the fluid warm at idle in neutral and it was about 1/8th above the low mark. I filled it a little and the level came up to a hair above the full mark. I tested it and everything was the same except manual first now had a thud into place when accelerating but that only happened a couple times. I readjusted the fluid level to a little above half way between marks and now the thud in manual first is gone but Drive and 2nd are still slipping.

I've ordered some line pressure testing equipment and it'll be here in a few days so I'll be able to perform some tests then...

What could this be?

Forgot to mention there are no leaks and that it was working perfectly at first.