I had 2.45s in my 68 dart 4dr with a 273 and it was fine and my grandma had an 80s fulsize with 2.94s and quite a bit taller tires and the 318 was just fine in there also. Either way I think I will be OK with the gear ratios, this is not a hot rod. If nobody buys my cudas 318 it may end up in here some day and it has lots of tq at low rpms.

As far as the swap thing am I understanding right that one of the 2.45 SGs should bolt right in?

I am not causing global warming, I am just trying to hold off a impending Ice Age!