Overdrive transmission

A500/RH42 or A518/RH46 both are good options, the gear vendors unit hasn't got nearly as much of a step going into O/D...
As far as whether to choose an A500 or an A518 if your making more than 450 HP I'd definitely go with the A518 but if less than that either works but I'd go with the A500, it's a little smaller so it fits better, it's a little lighter which is a good thing & most important it's got a deeper first & second gear... It also uses less HP..

A500 A518
1st 2.64 2.45
2nd 1.54 1.45
3rd 1.00 1.00
O/D .69 .69

What that means is the A518 would need more gear to launch the same as the A500

A500 with 3.55 gear 1st gear is 9.37
A518 with a 3.55 gear 1st gear is 8.69
A518 with a 3.91 gear 1st gear is 9.57

So to match the launch of the A500 the A518 needs nearly a 3.91 gear and honestly to make up for the HP used probably every bit of a 3.91.. Which hurts the highway RPM's... Though honestly not enough to be overly concerned... If you have an A518 use it.. But if you have both I'd look real hard at that A500..