The small block 350 uses a smaller main bearing diameter than the Small block 400. To run a 400 crank in a 350 the crank must be "turned" down at a machine shop to fit the 350 bearing diameters. You also will need to use special pistons with the correct compression height for the 350 IF you are using the 5.7 inch 350 connecting rods. If you use the 5.565 inch 400 connecting rods , then you can use regular 350 pistons. The longer stroke 400 crank also requires that you provide additional clearance inside the block, by grinding notches to eliminate the crank hitting the block and other parts. Finally, the entire engine must be re-balanced to make sure that the engine does not vibrate. The 400 is externally balanced and the 350 is internally balenced so you need to use 400 flywheel and 400 harmonic balancer.