need help and some info on engine build

[mikey2409]

I'm currently rebuild a 350 with a 400 crank and running into issue not having a clue that this engine had a big crank so heres some help I'm needing to determine and fix my problem first I know I need to build the top end with a different cam which means need to change springs lifters and so on and my next thing is I'm going to be running a th350 so do I have to upgrade my flex plate to a 400 or can the 350 flexplate work beside up grading the torque converter to a higher stall