Rear differential upgrades?

The 390 has plenty of torque to work with taller gearing than a smaller engine. I had 3.25 gears in my first car with a C4 and 351C-4V. That was not too much gear for me at all, I think I'd try the 3.50 gear unless you need lots of highway driving time. 

I have 373's in my 1998 Explorer, and in 3rd gear(1:1) it runs 3000rpm at around 60mph. Your car will take shorter tires such as 26-27" I'd guess, so the same gearing should push yours over 3000rpm @60mph.

mrbpatsfan wrote: Hello fellow enthusiasts. My 68 Torino GT came with a FE 390, C6 transmission, and an open differential with a 2:73 GR. The GR is way too conservative. I’m thinking limited slip with 3:25 through 3:73 for around town with 15” tires. Any why nots based on performance? Thanks in advance.

what performance level are you feeling? you don't mention highway at all but comments all reflect the inevitability of at least some highway time happening. your 390 should not be a slouch in the TQ dept with a 3.78" stroke, that attribute can be used to run a slightly lower numerical gear than say a 289 with a (can't get off it's ass) 2.87" stroke. have you had a play with a gear ratio calculator? or run the car up to speed in D2 and see if 60 mph at that engine rpm is a no-go. do you have a tach? if no maybe get a cheap POS and wire it in temporary just for finding your rpm tolerance threshold? 

How Fast Do You Want To Go?

Totally agree with CJ, the most accurate way for a person to know the "actual" axle spline count....unless of course you can't count and/or remember .