I totally agree with everything that Dave said.

A couple of other points to consider.

Unless you go the stroker route, you give up 50 cubic inches.

The the 77 and up 351Ws used small valve heads (same valve and port size as the 302s). Finding earlier 351 heads and having them properly rebuilt, or upgrading to aftermarket heads is really needed to build a competitive motor.

When you start adding up the parts to build a 351 to a similiar power level as a big block, your likely better off with the big block.