273 or NOT 273?

I don't think it would be crazy hard to build a low 12 273, what's that 380-400 hp ?

First main obstacle is cylinder heads and valve size, well good thing 400 hp doesn't need that much of a head to get there. 66fs runs 1.88 valves no problem in a 273 bore with a little notching probably fit magnum valves size. So choices are port one of 273/318 heads with 1.78-1.92 valves or run 360/5.9 heads. I'd probably mildly port out a set of aftermarket replacement magnum heads.

Besides possible custom pistons, the rest is a fairly straight forward build cam gearing and stall choices are gonna make or break the build.

Ballpark say a 282 making about 1.2 lbs-ft per cid, = 340 lbs-ft peak say making about 90% of that at peak hp (306 lbs-ft).

400 hp x 5252 / 306 lbs-ft = 6,865 rpm, 400 hp @ 6,865 rpm and 340 lbs-ft @ 5500-6000 rpms. Obviously the better the torque (VE%, CR etc,,) the less rpm needed or vice versa.

Question is, is that what you want ? Combo wise it shouldn't be much different than any other 400 hp build.