Ultimate A-500

The A500 has ratios of 2.74-1.54-1.00-.69 od. The splits are .56-.65-.69
This is a nice progression, but they are very wide, causing the rpm drops, when at WOT, at the shift to be quite far. It also means that the trans wants the engine to carry the Rs quite far past the rpm of peak power, to drop in at a similar power level.
A 400hp 360 is about a [email protected] cam and hi-flo heads.(Or a 240cam with stockish heads).For fast times,the 230* will want to be shifted at 6500 or so on the 1-2; On the 2-3 it will be about 200/300 lower.
That means your 3.91s will run out at about 46mph , and then on the shift, the Rs will drop to 3600. It's a good thing you have a 360, (and I hope you have lots of compression) to pull the Duster up out of that hole.Second will take you to 80.
If you wanna be fast, you are kindof wasting a lot of gear here, cuz the speed limit is probably not that hi, where you live.But the cruize rpm will be about 2180, so that's just right for a 230* equipped 360 .
To trap well at 60mph, the gear of choice might be 4.56s. These will get you 60=5500 which is about right on for that engine. These would cruise at 65=2540(zero-slip).
At 35mph in second, the 3.91s will be at 2750, while the 4.56s will be doing 3200,unless of course you have a higher stall TC. That extra 450 rpm will make the D really jump.
Personally, I woulda gone with better heads,the next smaller fast-rate cam,the 3.91s and a 3200TC. Maybe you did too,lol.