If you're going to RWD you'll want to set the engine farther back into the vehicle for better weight distribution. A Miata trans isn't that bad. I will say that my 6 speed (01-04 NB) has very close first and second gear ratios but everything after that is fine. I'm sure I could wind out the speedo if I wanted to but I don't have anywhere to do that legally. Are you going to tub the rear end? What's this car being set up for? Drag racing? Drift? Or just making it RWD for the hell of it?