3.91s in my 340 now!

[fishmens67]

When I was married in 1980 our weekly commuter was my 67 fastback with 383,torqueflite 3800 stall, fenderwells and 4.57 with 26" tire.
Fast forward 45 years the same car with 451 4800 stall and 4.10 28" tire

 

[Illahe]

I also have a 72 340 Duster that originally had 3.23s and have swapped to 3.91s. Although my 340 is now 422” and I run a 4 speed. In my opinion 3.91 and a 4 speed are a match made in heaven for a street car.

3.91 gears are a good balance of ratio and MPH, but the tire size will play into your 1/4 mile time. I run those gears with a 365 4spd and found that a 26" tire is a handy cap for mph. 3.55 gear ratio pulls a better mph by 3.5 mph but a 1/3 second slower et.