What stall speed do I need

Not sure what the cam is, Mike at MRL Performance is building the engine
It's a roller motor with alum heads, 10:1 compression

I was using a 3400 currently and a guy told me I may want to go with a 3800 with the new setup. I just wanted to see what others might recommend.
I know for sure I won't go lower than the 3400, but if more will be better I will go up

Stall depends on the TORQUE of your set up. Put an off the shelf 3000 stall on a 700HP/650tq set up I bet it will flash to 6000 Rpm.

I would probably try what you have first. I bet you get 4K Stall out of it with your new set up.

What brand converter is it and what size is it that you have now?