First, a 3.75" crank is a 400 c.i.d. It has a 3.75" stroke. It is also an external balanced crank where as all the other small blocks are internal, so you will have to have the flywheel and balancer for a 400. The mains will have to be turned down to fit in the 350 block since the 400 has bigger seats. The counter weights on the crank may hit the oil pan rail on the block but that can be taken care of with a small grinder very easy. You can either use the 5.7" rods or 6.0" rods as they will both fit, just make sure you have the right pistons. I have had mine ballanced so there is no problem with the rods hitting the cam or anything. Good luck.

Kodiak