No need to go bigger valve size in factory 273 heads, head swamp with bigger valve heads & notching cylinders unless ya want a high rpm beast that will require a 4.10 & up rear and preferably a 4 speed which is what I built for my Dart. The factory head design is not anemic on a 273.What is anemic are the factory exhaust manifolds. Ditching the logs of doom for headers will get ya from a low 16 sec quarter mile time in stock 273 commando car to high 14s with just that mod. Just leaving stock valve size ,getting a good port job on heads, adding a port matched intake manifold like offy still offers plus a little more cam, decent carb and of course headers will open these little fellas up like a Christmas present.