American power train offers A body 6 speed manual

The T56 isn't overkill at all, especially with the close ratio box-

First Gear Ratio: 2.66:1
Second Gear Ratio: 1.78:1
Third Gear Ratio: 1.30:1
Fourth Gear Ratio: 1.00:1
Fifth Gear Ratio: 0.80:1
Sixth Gear Ratio: 0.63:1

Compared to 833's-

1964 thru midyear 1966 A-body: 3.09:1 - 1.92:1 - 1.40:1 - 1:1

1964 thru 1970 B/RB engine: 2.66:1 - 1.91:1 - 1.39:1 - 1:1 = (The "close ratio" V8 gearbox)

1970 thru 1974 TA: 2.47:1 - 1.77:1 - 1.34:1 - 1:1, = 1970 T/A & AAR 340s

Notice that the T56 close ratio box is very close to the gear splits in the T/A 833, with the exception of 1st gear. But even that isn't too far off. You already see where I'm going right?

The issue is, AJ stopped all his calculations at 85 mph. Yes, it's true, you shouldn't ever find cause to go faster than 85mph on public roads. But if you're not talking about public roads, you have a whole new ballgame. Like if you want to take your car out on a road course a couple times a year. Not to mention he did all his calcs with 28" tires, which is a street/strip or strip only kind of height. That's a drag race tire.

Run the close ratio T56 box with 3.91 gears and 26" tall tires. The result is perfection, if drag racing isn't your only goal and you're ok with the 1st gear ratio. Which you would be if you're an autoX or road race guy, because 1st gear lets you do 20 to 40 mph between 2,700 and 5,400 rpm. That's pretty darn good for most autoX courses, as they generally try to limit you to ~35mph. Keeps you right in your powerband, even with moderately hot cams.

And really, how many people have cams that start above 2,500 rpm? The T56 with the .63 6th gear, 3.91's, and 26" tall tires puts you at 2,500 rpm at 80 mph, which is not an uncommon speed to go on California freeways. In fact, it would be perfect for cruising down I-5 to the Spring Fling festivities, not to mention even better at Willow Springs, when the T56 will theoretically let me hit 160mph at 5k rpm. Which it won't because of air resistance, but again, perfect. I can probably hit 150 mph on the straights before I hit redline, even considering aerodynamics. So yeah, I probably won't use 6th gear a lot, most freeways could be handled with 5th gear especially on shorter trips.

But here's the thing, you have to cut the crossmember and the tunnel for a 5 speed TKO anyway. And the T56 magnums that American Powertrain sells will handle 700 hp, so it should be good forever in most street cars. And it really doesn't cost that much more. I've priced out my conversion for under $5k, which is all new parts through Summit racing. The only thing it doesn't include so far is the crossmember and tunnel work, but US Cartool is working on a conversion crossmember and tunnel for A-bodies as we speak, which should also work for torsion bar cars. For the amount of work it takes to get the TKO, you might as well go T56 and get the extra gear and additional strength. Even if you don't ever use 6th.

Also, you don't have to do the math. There's a really handy gear ratio calculator at vexer. This is the non-flash version of the one that gets listed all the time, but I use it because it's set up for a 6 speed.

http://vexer.com/automotive-tools/speed_nf.php