Rebuild or replace?

Ima thinking as a cruiser, low rpm performance might take precedence over absolute power, and so torque at ~2050 stall (about the factory rating) is what's gonna make you smile.
A nice number for this 3.55 combo, might be .060 to .070 foot pound per pound of vehicle weight. So for a 67 GTconvertible at say 3100 plus driver and at least one passenger, cuz who cruises alone ( I mean besides me, and I ain't really cruising, perzactly) so say 3450 loaded with a bit of gear, that would be a minimum of 207 ft lbs at the factory stall of ~2050. Can the 273 do it?
If less than that, it's gonna be lazy.
The number I'm favoring for this combo is a minimum of 1800 ftlbs @ stall, divided by the Torque Multiplication. So with 3.55s and a 2.45low, that makes 1800/(2.45x3.55)=8.70.
And 1800/8.70=207 crank ftlbs. This will squeal a pair of street tires, but not spin very far. However, it won't be dreadfully slow.
Sticking with this method, the next step Ima thinking, is 10% more. This could be done;
by engine;so 273x1.1 equals 300 cubes; or
by increased torque,so 207x1.1=228ftlbs crank at 2050stall
by stall;so 2050 plus an easy 200=2250,or
by gears; so 3.55x1.1= 3.91s
At zero mph;
of these, the gears at zero mph are the least effective.
Unless you have killed the low-rpm torque with too big a cam you can't do much to increase the torque at 2050 rpm.
That leaves cubic inches and stall. if you can buy a good used bigger engine for the same price as a good convertor, that is the better choice. But if you want to stick with the 273, then a higher stall is your only choice.

If you continue this way in 10% steps, pretty soon your stall is gonna run off the scale.
But engine wise; the 318 is 16.5% bigger, and the 360 is 31.9% bigger, and both will pull that 2050 TC very nicely.
The 318 will be happy with 3.23s and the 360 with 2.94s, both to make the 1800minimum ftlbs at zero mph.
The 318 would need 1800/(2.45x3.23)=227 crank ftlbs at 2050, which is a piece of cake for it.
The 360 would need 1800/(2.45x2.94)=250 crank ftlbs, which is also a piece of cake for it.
But notice that in each case of reduced rear gears, the hiway cruise rpm goes down about 9%.. So with 27 inch tires, and at 65mph,, the 3.55s will cruise at 2870@ zero slip, the 3.23s at 2610, and the 2.94s at 2370. That last one is 500rpm down.... yet with the 360,takes off with the same performance as the 273 with 3.55s. In a weekend cruiser, at least to my way of thinking, this is a good thing.

Finally if you have to freshen your engine, it costs about the same no matter what SBM you start with.

Ok waitaminit, I once installed a complete 318 top end and cam onto a 340 bottom end, with nothing but headers and dual exhaust. The 340 was the early hi-compression variant. That 2bbl made a smoking hot cruiser.