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

Best to keep it. They are cool little V-8's that rev high. 4.5L is larger than many current very expensive V-8 Euro cars. Check compression first. If you do take apart the engine, check the pistons and bores to see if a simple re-hone is possible. A new commando piston set is ~$400, so that might be the time for a "push-on vs swap out" decision.