273ci thoughts?

The reason 273 and for that matter 340 are considered the revving small blocks is cause they both have the higher head cfm per cid than 318 and 360, 340 have the heaviest pistons compared to the other 3. You would need a set of 360 heads on a 318 and ported 2.02 heads on a 360 to make them all have similar cfm per cid.

I don't consider 273 to be an hp underdog unless we're talking 500-600 + hp NA, A set of stock Magnums on a notched block would give enough flow for almost all 273 builds.

The main problem is gearing, since most don't want to run more than 3.55, most moderate built engines will produce about 1.15 lbs-ft per cid so a 360 will be 414 lbs-ft and 273 will be 314 lbs-ft in first gear 2.45 times 3.55 times lbs-ft you get 3600 lbs-ft to the ground for 360 and 2730 lbs-ft for 273 huge difference even if both are making the same power. The 273 would need a 4.68 gear to put 3600 lbs-ft to the ground. Even worse if up against a 440, with 4400 lbs-ft to the ground the 273 would need 5.72 rear gears.

If willing to gear it build it if not go bigger, but obviously some have different criteria like /6 guys, I get it sometimes I think about pulling my 360 for 273 or even a built /6 or sometime 500+, just be informed of the plus and negatives .
Generally the old saying no replacement for displacement hold true, even more rpm and power adders are just another form of increased displacement.