Still up in the air....

[jel111]

Really good points. I already have the rear end set up with the 3.90 rear end. What kinda top speed would I be looking at if I stayed with the six and the mono jet?

[jocko]

Originally Posted by jel111

Really good points. I already have the rear end set up with the 3.90 rear end. What kinda top speed would I be looking at if I stayed with the six and the mono jet?

Depends. If you know your tire size, and then insert your 3.90 ratio into a calculator (one provided below) (or experiment with any gear - or insert your old 4.11 for comparison), you can determine what rpm you'll be turning at any given speed. With 3.90, you'll still be hummin pretty loudly at higher highway speeds.

your current trans top gear has a 1:1 ratio (stock 3-speed & 4-speed manuals, TH400, TH350, PG, etc). All overdrive trans have a <1:1 ratio, thus the better mileage and ability to run at higher speed for lower rpms.

As an example - let's say you have a 235/75/15 tire - it's about 29" tall - and a 3.90 rear - based on the calculator, you would be turning 3160 rpm at 70 mph (whereas with your 4.11, you'd be turning 3330 rpm at 70). In other words, the difference between a 3.90 and 4.11 will almost be unnoticeable. Regardless of tire size - the relative (%) difference between cruising rpm will be the same small ratio when comparing a 3.90 to a 4.11.

http://www.carforums.net/rearcalc.php

Here's another calculator that you can use to calculate changes as the result of using an o/d trans: (this is also a great ref for what several different transmissions have for gearing) - for top end, only the final ratio and your rear end matters - http://www.angelfire.com/fl/procrastination/rear.html

So, as a comparison - with the same tires (29" tall) and same 3.90 rearent, a 700R4 at 3100 engine RPM equates to 97 mph!.... (so,. to your quesiton - your 6 cyl and monojet may preclude that...)
Or more appropriately put - instead of cruising at 3100 rpm to get to 70 mph with a non o/d trans and a 3.90 rear, you only need 2200 rpm to go 70 mph. Start counting the gas money $ you'll be saving now!

Anyway, hopefully these calculators will help you decide. Also concur with the above comment about a modern eng/trans combo transplant - very good point if you don't mind goin modern. Personally, COMPUTERS scare me.... (on cars, anyway). But it is a good option. A bit more of an involved swap, but many have done it before, so am sure you could also.

[Captainfab]

I wouldn't bother with switching from 4.11's to 3.90's. There really isn't anything to be gained. If you decide to go with the 396/TH400, I would change the rear gears to a 3.08. The BB will have enough torque to pull the taller gear, and it will be more highway friendly. If you stay with the I6, and install the 700R4, I would suggest a 3.42 rear gear. The lower 1st gear of the 700R4 will help get moving with the I6, and you will have a nice low highway cruise rpm in OD. Of course this is dependent on your selected tire size.

[1Bad62Pro/Street]

Being a Hot Rodder that I am I would say 396.
But you need to check to see how much it will cost you rebuild a stock 396.
It suprised me. And i was wanting to hop it up!
I was offered one once from a friend.... till I seen how much it would cost to rebuild it just stock, heads and all!