I have no idea how you figure that. The 1 and only advantage the 273 has is a lighter piston. (The potential to rev up quicker.) There is absolutely no reason ether engine would haves problem matching rpm for rpm.
There would be no reason to do so. The 318 has a 45 CID advantage get the 273. That’s HUGE!
This makes nearly 90% ZERO sense.
Dependent on the year 318 you start with.......
We will assume both engines are 2bbl.
If you stand by the 273 as a start with a 4bbl., then mentioning it above as a add on should not have been done but I’ll go there and explain it below.
Example! My ‘67, 318 (forged crank years right?) started life with a 9.0-1 listed/advertised compression ratio. Not bad. A 1 point increase is about, at best, at this low power level, 3% on a good day. If your lucky.
As you said, add a cam and matching springs, 4bbl. carb & intake, HP mod’s done at a 45 cube advantage.
If someone starts with a mid ‘70’s to late 80’s 318, the additional money spent on milling the heads and intake for the cheap route to go from 7.8-1 to 9.0-1 is a worthy expense. Late 340’s also suffered with low ratios as well as always with the 360.
Truly, all of this is about a 99% wash because nearly any as build completely stock engine from the days gone last will need a total over haul.
At least there are off the shelf forged or Hyperutecic pistons or the 318 available in flat top or some as well as various dish depths and numbered valve reliefs. That can not be said for the 273.
OH! You crap on a cast crank even though there known to handle 500 hp/tq easy. Are you going for a 273 @ plus 2hp per cube for 546+ HP?
From a durability stand point, I have never seen a crank last long no matter what the material is being used when the engine is beat on and/or suffers from a lack of oil pressure. Every crank dies when that happens.
There is no dispute on stronger materials lasting longer.
It is to bad you can not (for the most part I have seen! LOL!) turn a 318 into a 340. Even going to a 4.00 bore size is dicey. Possible? Maybe. But at what cost to the thickness and integrity of the cylinder wall? if I can take one safely to .060 (327 cubes is it?) I’ll go there, if the cylinder walls need to go that far and can still have sufficient eat left over.
The ability to “Rev” an engine “High” has a few elements to it. IMO, it starts with proper valve control with the right spring.
