"Best gearing" is 4.30s AND with a 2800 stall, and use the family car for hiway.
But yur gonna need at least 275/295 tires.
But I gotta tell you something.
4.30 x 1.45= 6.24 ratio in Second and
2.76 x 2.45= 6.76 ratio for First.
The engine doesn't care how it gets these ratios.
These numbers are so similar that Second gear with 4.30s will feel almost identical to First gear with 2.76s. Therefore, this is trading away a really good cruiser gear (the 2.76s) for a really really good starter gear (the 4.30s)
IMO, the better choice is a 2800 stall and leave the 2.76s alone. Then install the 275/295 or bigger tires, and you may not need the Suregrip..
If yur not willing to install a 2800 Convertor, then
I agree with all those who point to from 3.23 to 3.73, will get you started.
-----------------------------------------------------------------------------
IMO, you need to come up to my place (~13 hours each way), and spend the night. The next AM, we can go to the shed/shop and dig out, everything you need which is a mid 70s A904 cloned to be the same as from a 71 Demon and then hotrodded, and it already has the 2800 in it.
Additionally, I can supply a preassembled center-chunk from 2.76 to 3.23, with a reconditioned Cone-style SureGrip. and
I can have a pair of New 295s mounted up on a pair of aluminum 15x10s, on a narrowed rear end, that should slide right under your Duster. I can even supply a pair of matching wheels for the front/no tires at this time, but could have ready by the time you get here.
Then you can take it for a spin to the drive-in at supper time, about 18 minutes each way, and see how you like it. Then we can sit around until bed time and you can stay in the spare bedroom again. But if you don't like something, next AM, we'll just back it into my little shop and change it.
I'm retired now so you can come anytime, stay as long as it takes.
Or if you're into 4-speeds, I have a complete kit (less the floor hump and less the under-dash reverse light) that I might let go. This works well with a set of 3.55s, that I have, lol.
Yeah, I know, you only asked about gears, lol.
