If you get CenterLines instead of Welds you won't need longer studs.
I'm not sure exactly what setup you need for IRS but for the fronts I run a 15 x 4 Centerline Telstar (I think the BS is 1.75 but it don't matter cause they only make one size in a 5x4.5). If you don't want to grind on your caliper or get longer studs you can get a 1.25 inch spacer with studs on ebay for $55. The tires will stick out a little but but it's not that obnoxious.
Otherwise I think you can get 3 inch studs and a 5/8 (I think) spacer and grind the shit out of your caliper.
Also, PHP has a kit to convert to GT front brakes for $250 but you still end up with a 1/4 spacer and longer studs.