I think the 5.13’s will get old really quick, if you drive the car often. I’d recommend trying 3.91 or 4.10’s first and see what you think. At least gears are not as bad to change out in an 8¾” as other differentials are.
I have 5 different 8¾” center sections in my garage, each with a different ratio, that I collected in the early ‘80’s (when they were a dime a dozen) for experimentation in my ’67 Plymouth Satellite (which had a nice running 340).
Actually, I'd start with a 3.55's, but if you are already thinking of 5.13's then trying a set of 3.91 or 4.10’s might not be a bad idea.
The A/C has a few more things involved.
The cast iron RV2 compressor (aka: the boat anchor) is bullet proof but also weighs a ton.
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The aluminum C171 (or aftermarket Sanyo) is lighter and takes a bit less horsepower to rotate (when on).
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This is a '73 Body with a Sanyo (aftermarket) kit installed. This is a popular kit, if a person doesn't want to look for OE or original parts.
Either system does add weight to a car as well as rotational mass (pulleys, clutch, belts, etc.) so adding A/C might not be good if racing the car.
For cruising, then (again), it depends. I don’t like sweating so, it is worth the effort to convert, in my case, my ’77 factory no-A/C wagon to A/C.
I will say it is a lot easier to convert a A/C car to non-A/C, than the other way around.
Other considerations are there will be more heat at/by/through the radiator – so an upgraded cooling system is highly recommended. In my case, replacing the 18” (or 20 or 22") radiator to the larger 26” version (if you don't already have the 26" radiator to begin with. My current 18” radiator can’t even keep my car from pegging the temp gauge in 100’+ F (38’+ C) as it is.
’76-79 F and M body A/C dash systems (evaporator cases/firewalls) are considerably different from the ’80-89 FMJ dash systems. If a person is considering converting a ’76-79 factory non-A/C car over, I would use the ’80-89’s Evaporator system.
What age is drivers permit/drivers license there?
In Oklahoma, kids can drive at 16 and get permits at 15½ (drive with a licensed driver in pass seat). My twins are now at 12½ . . .
BudW