auto trans for 1965 273

The 273 has a smaller diameter register/counter bore in the crank. A special custom torque convertor solves that.

True and correct.

The 904 has a much more compatible lower gear ratio.

The 727 and 904 gear ratios are the same:

2.45:1 (first)
1.45:1 (second)
1.00:1 (third).

Except the '80-up wide-ratio gearset for the A904/998/999 is 2.74/1.55/1.00, but that doesn't enter into this topic.

(and as others have noted, the 727 is unnecessarily heavy and will consume more power than a 904, and the truck trans is way too big to fit in an A-body without major surgery.)