Go with the 400. That longer stroke is going to move you a lot better on the street. As far as the flexplate/flywheel goes, several manufactures have bolt on balance kits that bolt between the crank and the flexplate/flywheel so you can use your 350 (or smaller SBC) flexplate/flywheel on the 400 if you don't have one made for the 400. Speedway carries one and I've seen them advertised in several rod magazines also. JMO.