There's many things to consider including what transmission you are using. If you are running an automatic you will want to consider what engine speed you will cruise at on the highway as to what stall speed converter you get. You need to know the final drive ratios and tire diameters to figure this out.

In general the 370 is a good all around ratio I would say. I set my deuce up to pull 2700 rpm at 70 mph (30" tire dia, 350 rear) which is right where the converter stops slipping.

I am very skeptical that an older 454 caddy gets 24 mpg. My '97 SFI 454 truck gets 11 mpg at 80 mph. Albeit the truck is heavier and higher drag, the 24 mpg number sounds optimistic to me.

Regards, Kitz