I converted a Rifts game to d20 using 2 rules iterations. First was DragonStar. That was great because it has all the magic rules of D&D intact plus high technology. Second was Omega World. I loved it. OW added all the freaky mutations, which I treated as psionics (there were no spellcasters in my game as they were playing Coalition soldiers). Also, OW treats all the tech items very simply.
My players hated it. By and large, they couldn't get past what they perceived as a loss of statistical power in the conversion. They didn't see that the entire framework of the world vis-a-vis the rules changed. In other words, they couldn't see that they were just as powerful as before since everything else in the world became relatively less powerful, too.
Anyway, if I did it over again, I would use the OW rules with some DS classes. Use MDC all you want. Rifts always seemed to me to be Palladium fantasy, Heroes Unlimited etc. thrown in a blender anyway; so the conversion of d20 D&D to a high-tech Rifts world should be pretty easy.