Performer RPM Cylinder Heads For S/B Chrysler

Just put 66 up heads on it, problem solved. These have the bolt angles standarized, so anything that fits on a SBM will fit and bolt on.If you swap to the later heads, you have to choose the closed chamber type to preserve as much compression as possible.
But I gotta warn you; if you don't do something about the dynamic compression ratio, bolting a 4bbl onto a 2bbl 273, is IMO almost a waste of time.

The low cylinder pressure can be partially offset by a bigger TC and performance gears.
But this will end up costing you almost as much as proper hi-compression pistons, and will cost you more money per mile driven in terms of fuel-mileage, and without the cylinder pressure. the engine will be a bit of a dissappointment.

In all honesty, the cheapest way to power is a bigger engine.

Here's a mental test, for your 273 2bbl factory stock engine
Draw a flat line 5 inches long. Mark the left end 0, and measure off every inch.On the right end of the line make a mark 1/4 inch above the line. Connect that dot to the 2 inch mark.Next make a mark on the flat line at 4.5 inches, and draw a vertical line up and across both lines. Erase everything to the right of that vertical.
Now imagine that the flat line is the WOT power of your 273 2bbl........and the sloped line is the power that the 4bbl will add, and every inch is 1000 rpm.

Ok now, under the 3rd mark from the left,write 33mph, and at the 4inch mark, write 44, and at the 4.5inch mark,write 50mph . These are the mphs your car will be doing with 2.76 gears. Notice that at 33mph/3000rpm there is only a tiny horsepower gain. And the most gain will be at the right end of the line, which is now 50 mph.
See what I mean about a 4bbl being almost a waste.
I can't tell you what the power difference in that sloped line is, but at the right end, at 4500rpm,at 50mph; I doubt it would be 10hp.
This is just an exercise.

Here is another exercise
Draw a parallel line 1/2 inch above your 273 flat line,and 5 inches long, to represent a stock 318.
Next go up another 1/2 inch and draw another flat line, but 6.5 inches long, to represent a 360.
Next , draw a diagonal line to connect the zero mark on the 318 line to the 6.5 inch end of the 360 line, to represent a 340.
The 360, represented here is not stock, but has all the 340 goodies on/in it
These are just representations, and imply no accuracy, and only hint at power increases;just exercises.
Consider about 20 cubic inches as one step up in engine size. So from 318 to 340 is 22. and from 340 to 360 is 20 and so on. But from 318 to 273 is 45 cubic inches,over 2 sizes smaller. Just an observation. They work very well in the lightweight-As they came in, having the same or similar cubic inche to weight ratios, as the regular-As with 318s.

I see you're getting ready to spend some hard-earned money on your combo, and I'm just afraid that the end result might be disappointing.
To put a cam in her, to be effective,will require the cylinder pressure to be brought up as well, else the bottom end will be very disappointing. The roller conversion thus will quickly exceed $3000, and you stil don't have heads to go with that cam So now its gonna be $4500. And since you went this far, you're gonna need that 2800TC and 3.55s or better. Now you're up to $6000ish,before bolt-ons, which can easily add up to another $1200
And the saddest part is what did you get for your $7200? I'll guess 40 to 50 hp, or about what a stock Magnum has......which you might find a low mileage unit of, for a fraction of that.
Now it might sound like I'm dissing your 273, but this has nothing to do with her. It's all about how much power per dollar do you have to spend.
When it comes to labor, it costs the same to build a 360 as it does a 273, and you are instantly 87 cubes bigger,instantly. Even if the 360 is a 2bbl, this is likely to be over 60 horsepower more.
When it comes to parts, it will be cheaper to build the 360, on account of the pistons, and ballancing. And the 360 won't need the 2800 nor the 3.55s, nor headers nor even a 4bbl in an early-A.
And if you can find a Magnum, you already have an excellent platform to work with. Sock for stock, it's hard to fault a Magnum.
So let's say you do find a low-mileage 5.2 with log-manifolds, for $600 delivered to your door. She's gonna need in induction system and an ignition system, and that's the expensive things taken care of for about $1000 with all new parts. Total say $2000. And that 5.2 is gonna put out maybe 240 just like that,with a 4bbl,compared to your 273 which was rated at 180 IIRC. so that would be 60 hp for $2000, compared to 50hp for $7200. Which will you choose?
And as if that's not enough; a stock 360 2bbl engine will put out about the same 240hp, with a 4bbl and headers. But if you spend the 7Gs on it, you will be looking at 400 plus hp, maybe as much as 430, or 450 with a mega roller cam. So now its 250 hp for 7200. Oh hey, lessee 40/50 for 7200, vs 250/280 for 7200.
Just FWI, the 400hp 360 only needs a 230*@.050 cam with hi-flo heads, and is a very driveable combo. The next smaller cam will be dynomite.
So there are some things to think about. A 273 build can be a run-away train on a short track. The builds just seem to escalate and end in many dollars spent for, in a lot of cases, a disappointing ending; in terms of dollars spent to hp increase, or even to dollars spent to performance increase.
And just to drive the point home; once you get down to 10pounds per hp, first-gear traction becomes non-existant, and you will spend another couple of grand trying to solve that. So you might as well budget for that now,lol.