Main problem is getting the main journals ground to fit a 400 block. It really isn't hard to do, if you have a good machinist. The rods journals are already the same size on a 383, 400, or 440 crank. Then you may need to clearance the block in a few spots with a hand grinder. From there on, it's just basic engine assembly. I like to use the 440 rods with a 1.320 compression height piston. Then you end up about .017 below deck. Use a closed chamber head with a .020 head gasket, for good quench, and that thing will fly!
