273ci--a keeper in my 64 Dart GT 4-speed?

its hard to wanna build the 273 when it costs the same moeny to build a 318 , 340, 360...EXCEPT that the 273 pistons cost WAY more than the 318,340,360 pistons do...so in reality side by side it costs more to build a 273.

If you build the 273, the best bang for your buck will be porting the heads and running a matched cam, matched for your driving intention, power expectation, and head flow per lift.
There are plenty of really strong running 273's out there, but those are the ones with a good matched cam and some head work done, the rest are just a lil better than stock.

Stay with the 1.78/1.50 valve arrangement, have them ported to flow in the 195-200 cfm int 145 cfm exh, those heads will do that easy and all by .450 lift which is great cause you dont need a big cam with nasty street manners to make such a jump in power and will keep the mpg around where you want them.

fwiw you could up the cid to a 318/34/360 and all the power you want will be more easily attainable by simple means of cid increase/larger bore.

what would be funny is to say throw a 4'' crank in it and have some pistons made, then it would essentially be a 8 cyl 225 slant..lol tiny bore and mountainous stroke.