318 or 273?

Several reasons "you might" want to go with a 318

I never can remember for certain "when," 68? Ma changed from the small to the large register crank. Now, if you have a late engine (large register) you can get / make a bushing to use an early transmission (automatic) but it's more work and expense to go the other way

You should be able to pick up a 318 pretty cheap on CL or elsewhere. As above, "way" more parts available for them.

The only reason I'd build a 273 would be for a "correct" resto