273 or NOT 273?

If you want cheap, go 360.
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I'm not a 273/318 hater. But
I'm going straight for "swept area"; and I'm willing to detune my bigger-inch engine to match the power of a hopped-up 273, and then be able to drive nearly twice as far on the same amount of fuel.......... with a cam nearly 2 sizes smaller, all-the while enjoying the fatter midrange.
Now,
if instead of a 67A-body, you had said 66 or older, which are lightweight-As, something like 300 pounds lighter; I'd not preach the big-bore advantage, which in those, all just goes to tirespin anyway.
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Back in about 74, at age 21, I built one of those with a 340, even with a 318 top end and a 318 cam, she was a tire-frying torque-monster! lots and lots of tirespin. That car I built from laying-around-junk and left-over parts from other guys cast-offs. It turned out to be my Third favorite of all-time combos; maybe even Second favorite.
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As for 360s;
Mine went together with 40 over KB107s (502grams) that landed at .012 below deck... With factory bushed 318 rods, and the lightweight pistons, mine was easy to balance externally. With 028 gaskets, and .012 decks, the Q was 040, and with small-chamber, big-port alloy heads, she was good for 11.1 Scr.
I later reset the deck-height to .007pop-up, and switched to an 039 gasket, the Q going to .032. With a few cheap oiling mods, and decent valvesprings, the valve gear goes 7200 all day long. As a streeter, this set-up was ready for any decent street cam.
Speaking of cams;
My first cam was the Mopar 292/292/108.. She drove 5.6mph@650 rpm, with a starter-gear of 9.44, but it was a struggle. Even with a starter gear of 10.97, I just could not drive slow enough to parade it. Well I could but, at typical Idle-Timing, she just wanted to buck down there, which then needed a toe on the clutch all the time.
My second cam was a 223/230/110, This cam had the potential for over 30 mpg, geared 65= 1600rpm; and with a starter of 10.97, this drove 3.6mph@500rpm. This made just enough Power to get into the 12s on 245 street tires. This was my favorite cam.
My third cam, 230/237/110* drives 4mph@550rpm. This has gone 93 in the 1/8th, at 3457 pounds, at 900ft elevation, on 295 street tires. The Wallace Calculator makes this out to be 433hp. But this 230*cam is NOT easy on gas.
In it's final iteration,
it's now running with a Commando 4-speed, 3.55s, and a GVod, which is often being used as a splitter. I have tried seven gears; but on the street, that was just too much; Five gears is all I need. In the 1/8th, 4 gears is all I need. She is currently geared 65=2240..

With what I learned over the last twenty years with that engine, I now have a pretty good idea of what my next cam should be ......... but I doubt there will be a next cam, as the current 230 cam, just won't die! This engine now has well over 100,000 miles on it.

BTW,
with those alloy heads, right from the beginning in 1999, she has always run on 87E10 gas, even at up to 195psi.
BTW2
Well, but.... I'm 25 years older now, and I must admit that I could be happy with a hi-compression 273/318; BUT, with a tiny solid-lifter street cam, and that A833-Commando/GVod, is looking pretty good. I'd be looking for a gear to be revved out in, at ~60 mph. The math points to ~7.76Roadgear, Which could be 1.92Second and 4.10s, which hits 60@ 5880, sounds good.. Using the Splitter, that would be 3 gears/2 shifts. But cruising at 65=2600 in od.
But, I'm also thinking, that these days, I could be just as happy with a hi-stalled A500LU, and something like 4.30s; for 60=~5500 in Second, and 65=2400 locked up in od. But as you can see, this combo really has no useful passing gear on the hiway, so I'd be looking at, at least a 318, to use less rear gear.
But a Magnum top end on a 340 with a small cam, and less gear, would restore the passing gear; yeah I'm liking that thought. Jus not with the heavy forged pistons, nor with that factory 340 cam..
But if yur willing to go that far, you might as well just start with a Magnum, but throw away the factory cam. and, if it was me, I'd install some really small chamber alloy heads and crank up the pressure!
To me in my streeter, pressure and the right gear, is worth way more than over-the-nose absolute power....... Cuz think about it; most streeters, in most situations, are not going much over the speed limit, which with street gears, means yur onlybuzzing her up to shift-rpm, just once, cuz 5500 with 3.55s is gonna get you 80 mph, in Second (Auto).
So then, yur gonna need a lotta midrange torque, to give you that kick-in-the-azz feeling .... which points straight to cubes, or lots of Cylinder pressure. Cubes is usually cheaper than pressure, especially as you cross the detonation line near 165psi on pumpgas, with iron heads. So if yur stuck with iron, you might as well go 340/360 . But if you can afford alloy heads and your design can accommodate enough Scr, then a 318 can be used. But to get decent pressure out of an alloy-headed 273, is gonna take a lot of Scr, which with off-the-shelf pistons, is gonna be hard. and if I'm gonna fork over the dough for alloy heads, I'm sure gonna need a special reason to put them on a 273. .... cuz on a 318, I'm already looking at plus 45 cubes and an easy 40 hp, just on those cubes! never mind the torque boost from stall to ~3600, the range in which most of your city driving is gonna be. Forty hp, at this level, is like two cam sizes! That's a lotta fuel-economy sacrifice just to say you have a legendary 273.