Is there a simple/easy way to ID a 273 engine vs 318?

Well hey.... Though you never said exactly what this car is,
back then, except for the Chrysler and imperial branded ones they did put even /6s in C Bodies
Chryslers and imperials only got big blocks "stock" back then ...
So I have to assume and deduce from the clues you do give that this is a Dodge or Plymouth branded C body... so why not a 273? If the car in question was originally a 318, a 273 would at least bolt up the same where a /6 wouldn't..... We've all been there, need an engine to get a chassis mobile and take what's available at the time just for that sake ....and a 383 wouldn't bolt up to a SB trans either ( using what you said above)