I say 351 buy a decent re built short block for about $2800. With balencer .That's close to the same as on 3 kit with extras. Give or take a few hundred here on either side .That 351 should run flawlessly with a great amount of street power and be quick of the street light .
A 351W with GT40 heads (if you already have them, don't hunt for them with this project) will make an easy 300 hp at the crank. 8 psi from an On3 will make that into 450 hp and the block will be plenty strong at that level. Put almost any decent aftermarket aluminum head on it and you will jump at least 50 N/A hp and you end up well over 600 hp on 12 psi with pretty much the same reliability.
Well I have a spare set with 1.7 rockers.So if I'm pulling heads on a 302 to put arp and head gaskets I'm sure as heck throwing on some 40s.
Since you have them, use them. You can probably hit 1 hp/cubic inch naturally aspirated (in boost friendly form). So let's say you have 350 crank hp. Add a mild very pump gas friendly 8psi and boom you are in the 550 horsepower neighborhood for reasonable money. With iron GT40's (from what I've read) conservative is the path to longevity. Conservative boost, compression, ignition, and safe fuel delivery, ie...don't run near the edge, and you will make good power without lifting the heads.