No, that overbore thing meant larger than the largest factory overbore of that engine family. A factory 340 or 360 is fine for a 273 car.

As for exhaust, thats a good question. The class is for high performance cars with a minimum of a 4 bbl carb and dual exhaust... Thats a question for the FAST tech guys.

You can run any axle offered by that manufacturer. Same goes for transmissions as long as its no more than 3 speeds for auto or 4 speed for stick.