The A500 has the lower ratios, for the best of both worlds, but
at your torque level, I doubt you would need them, so the 518 is still a good choice.
350/400T are excellent street numbers for 3.23s in an A-body.
In a B, I can see a lil help from the stall. But with the A518, you can run pretty much any gear that you want, until first gear becomes useless. Since you already have it...........