I have put a 340 top top end (heads, cam, intake and carb) on a 273 since I had a forged crank, TRW forged 10.5:1 piston shortblock. Pretty much ran like a 340, minus a small bit. The problem with a 273 and really high flowing heads you get past a 7,000 rpm peak. Not really friendly on the street, pretty much driving around in 2nd gear unless you are on the highway.