Changing rear gear in 86'Trans-am from 273 to 373 any issues?

Im changing the gear in the rearend to give me more take off ,its just to high geared at the moment,anywayz I would like to know if any1 has done this be4 and if there is any thing I should look out for or some helpfull tips... I am my own machanic I just finished building the 6.6 liter engine...its all together now but way to high geared with the 700R4 Trany !, I bought a 373 gear Kit to fix that I already know to use a marking compound on the gear thats in it now to see where gears jion or ride, but stuff like can I use old crush sleeve or how much backlash to allow so that I dont have a roreing sound or strip gear due to 350+ Hp.... I will also be adding a eaton Gear Locker to make it a positive rear instead of Limited slip! I just need to know more about possible shimming on the gears.. this type job is a 1st timer 4 me... I have the complete gear and shim/sleeve/seal KIT its a 7.5 rearend swaping the 273 out and putting in 373..... people have told me not to take out old crush sleeve and not to shim it becuase the 373 gear matches up to the old 273 setup I just need more info be4 jumping into it...any suggestions would be nice THX!!!