273 Numbers Matching car. Should I rebuild or swap engine?

I've rebuilt my 273. 4spdragtop is right, it's cheaper to rebuild something other than the 273. Pistons are pricey, you won't find an accurate head gasket unless you special order a pair from Cometic ($$$$$$$!). I probably wouldn't do it again. There's links here on building up 318's, I think there is a link on a 300hp 318 that Hot Rod Magazine did here on FABO.