Engine stalls in gear

I have rebuilt this whole 360 and have brand new parts too to bottom I have a rebuilt 904 transmission with a tci 9 inch converter I am finally ready to get her on the road and for some reason every time I put it in drive or reverse it stalls. I can have the engine running perfect for 20 minutes but as soon as I put it in gear it stalls please if anyone has any input please help me.