318, 340, or 360?

The problem with building smaller like the 273 is ultimately gearing. Obviously the bore is a problem at a certain hp level only get so much air flow but I'd imagine some one crafty could get 500 hp +. Then next is rpm 400 hp depending on the build would be high 6000 to low 7000's, 500 hp would be over 8000 rpm.

And even if that was ok with you back to the gearing problem. A 400hp 360 probably needs 4.10-4.30 gears to get the most out of it but most would run 3.55 and few 3.73-3.91. A similar 273 is gonna need about 1 ratio deeper so minimum 4.56. Even a 3.55 geared 400hp 440 would be better than 360 cause the 3.55 would suit the 440 better.

If your willing to gear it run it, if not build bigger.