GSP with a 6'2" MW frame would probably grind out Silva, heck a lot of people think GSP could do that with his current frame + 15 lbs of muscle. It may not be the most popular answer, given GSPs lack of finishes, but I think p4p GSP would take it style wise via decisions on most of these guys at a same size. He's not the flashiest of these guys, but the one who's shown the least number of holes. He's great on the feet, extraordinary with his wrestling, extraordinary with his grappling and sub-defense.
Jones would be interesting as well without his reach advantage. Maybe he'd still be the best, the man is vicious and his top game is brutal, and standup is also very good. The question is would his standup still be as good without the reach.
Anderson has clearly shown holes in his takedown defense and wrestling. He's the most impressive currently because of his finishes and domination, but he has holes.
Cain I don't think would be nearly as dangerous at a lower weight, his finishing ability would be much lower and his chin has already been cracked multiple times at HW, I don't see him doing to great against a same size Aldo or GSP. Aldo would test his chin and probably KO him. GSP would match him cardio for cardio, wrestling for wrestling, but with superior standup. I see that fight going the same way as most GSP vs aggressive wrestler matchups.
Cruz, Barao and Mighty Mouse are not even in it IMO. These guys are great but nothing about their skills is elite compared to the others. Their wrestling isn't on NCAA level, their striking is plain jane boxing and their BJJ skills are average-above average (no ADCC / tournament level creds).