Figure the difference to be just the cubic inch gains. If you get 1 hp per cubic inch, then you would be 49hp/cubic inches ahead of the next guy. It really depends on the final compression of your new pistons and new heads. You will also need to buy 351 specific headers to fit the fox body, because of the higher deck height. However, if you step-up to a 351w now, you will be set to run a nice 408 stroker down the road.

Also you can get the cheaper 351 Thypoon style intakes for like $350 off ebay and up to $650 for a TFS or Eldebrock.