Rebuild 318 or go Crate

If you go machine shop buy your own parts on line, they're less expensive and not marked up by the shop.
when I was in this same situation 6 months ago I found the 318 "more" expensive to rebuild than a 360, because thats what everyone does so parts less expensive.
agreed x2 on the headers - unless it's a looks thing you'll probably never see a hp advantage with $750 headers. The Eddie heads - well i want some too- for my drag car. some properly rebuilt 360 heads ($600-700) on a properly built 360 should EASILY give you that 300+ hp your search for.
Be careful of the "snowball effect" at the machine shop - one good part leads to another !
Also remember all of the expenses and the stress of the expenses will all be completely forgotten when all is said and done and you mash that gas pedal, the tires light up, your pinned back in your seat, and you looked over at your wife and she's holding on for dear life with a complete look of terror on her face!!
after that you'll never remember a cent you spent.
what size rearend do you have ?