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

It's all up to you. Really the easiest way to go would to rebuild what you have to Hi po specs (or what ever combo you choose) Hop it up too much and everything but the trans and driveshaft will need changed to take the power.
Check out my 273 build thread (link below) to give you a idea what is involved. Keep us posted on your progress. tmm