273 vs 318

Its not the size of your engine its the hp of your engine that matters. Eg 400hp is 400 hp no matter if it comes from a /6, 273 or even a 440 if there all setup for the best quarter mile time they will all do similar.

But bore is the ultimate restriction to power saying that 318/360/340 all have a big enough bore to make enough hp to break the block. 273 is a special case it is really to small. Anything over 300 hp becomes impractical but not impossible. Eg. 1.94 EQ's ported and milled for smallest chamber with domes and a big solid roller with the deepest gears you can get for your rear end probably get you well into the 500's hp wise but the next problem is rpm. A 273 has to rev about 1250 rpm more than a 318 and 1750 rpm more than a 360 to make the same power. And cause of the rpm (/torque) difference the main problem going 273 (or any small displacement) is gearing since a 350 ish hp 360 with 3.55 is basically the standard when it comes to A Body performance a 350 ish hp 273 would have to run about 1 ratio deeper than so about a 4.56 to keep up. Or to put it another way dropping from 3.55 to 2.55 on the 360 would be equal to a 3.55 geared 273. Since many don't want to run anything larger than 3.55 gears it really makes no sense to build anything smaller than a 360 even for a mild build (under 300hp).

To me when it comes to smaller displacement engines "if your willing to gear it build it" if not go bigger.
Your cubic inch should be around 0.9:1-1.1:1 hp:cid for a street car.