If it helps any, I considered the same thing (manual vs a500), and found a pretty good alternative -- the A833OD.
Now, a lot of people tell you to be wary of it because of its weak mainshaft and wide gear ratio spread, but those are things I'm willing to live with. I haven't installed it yet, so I can't tell you how my car runs with it, but I think it'll be good.
I'm running a very mild 360 in a 1968 Barracuda, and all I want is the ability to hammer gears and still get decent mileage. My car is very much a daily to driver that gets it's legs stretched between stoplights and on the highway, so close-ratio gear crashing isn't completely necessary. I don't know what you plan on using your car for, but just remember that there are overdrive 4 speeds.