What should the rpm drop be on a TH350 going from neutral into drive? Setup is a fresh 250 engine and fresh Trans from a local builder with a Summit converter-12 and 1700-2100 stall.

Im running conventional ATF and have not done the first service yet.

When I put it in gear the rpm drops about by at least 500 and it lurches forward like maybe the converter is not releasing all the way. The engine bogs down too so I have to set the idle at 1200-1300 or it wants to stall. Unfortunately, the builder has closed his shop but I may be able to track him down. Any suggestions are appreciated.