True but we don't know how much actual effect the bore size will have on flow. Haven't seen anyone do flow numbers a 318 bore nevermind 273. Did see one article on 350 vs 305 bore sizes on some Chevy cylinder heads and there was like a 10 cfm difference. I know 305 has a bigger bore than 273.
To get 400hp out of 318/340/360 the formula seems to be 200-220 cfm heads 10:1 cr and a 285 ish cam. With a 273 probably got to add another 20 cfm. The main problem for some would be at 400 hp you would need to turn the 273 around 7000 rpm.
If I was gonna build a 273 between 250-400hp I'd go with mild ported stock valve size magnum heads on a notched block with some 4.??:1 gears out back.
