[tzfbird]

If you put it in gear and it feels like doing a brake stand then the forward Sprag was put together backwards... I've done that before.
If you put it in gear and the engine revs like it's in neutral then you would be looking at a forward clutch problem. I would start by making sure the shift lever is in its respective gear according to the shifter/display.
If you do some searching regarding the shift solenoids you can also ohm them out to make sure they are good. If you have the pan off you could also remove and apply 12v to the solenoids and test them with air to make sure they are opening and closing.

If the one you put in is a rebuild your best bet is going to be a solenoid/valve body issue.

 

[tzfbird]

I completely missed the part where you said the sprag was probably not the problem. Sounds like you know more than I thought you did, sorry about that. Yes you are correct, what you said the first time didn't really register I suppose. I'm very new at transmissions as well. Sometimes as long as you know enough to point someone in the general direction for troubleshooting, it's still better than just telling them to go to a shop.

 

[tzfbird]

Do you still have the pan off? Go through and move any valves you can get to with a pick to check for one that could be stuck. Be careful though, I told a friend to do that once and he used a screwdriver which really messed up a valve bore. No need to do any more work than necessary.

There is always the possibility that a seal was cut on the forward piston during installation. Even the slightest cut can cause a problem. Can't completely rule out the sprag either, anything is possible. If you don't find anything after the pressure test I suppose you can just plan on pulling it again.
On the bright side at least you're not working on a FWD car. I had a problem with the transmission I rebuilt on my Grand Prix which I had to pull it a second time. That's 10 hours of labor just removing and installing that I'll never get back.

Good luck and keep us posted on what you find out.