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

If your 273 needs to be rebuilt, I would seriously consider replacing it with a 318 out of a late 80's vehicle. The 318 block and heads are virtually identical to the 273, the a 340/360 has slightly different mounts. Such a core 318 should have the closed chamber "302" casting heads, and built with KB167 pistons and a mild cam would be a nice update from the original 235 HP 273. If you plan on keeping the factory exhaust manifolds and single exhaust do go too nuts, they'll only support 250 HP or so, another argument against the 340/360.