I've built a couple sets, mostly with pressed centers that are 3.5" max backspacing.
you can put the flat centers farther in, basically all the way out to where the factory wms was if you get the correct diameter center cut out.
You wont be able to change the studs if you weld the center that far in, not that it is a big deal, you could always swap the studs for bolts if you booger them up for some reason, or get your centers made with "relief holes" for the studs to pass through.
Trail-Worthy Fab sells pre-made wheels, and will even powder coat them for you. but they start to get pricey when you add up the options.
8 Bolt wheels have flimsy outer shells, MUST have rim stiffener/reinforcement
12 Bolt (or newer 24 bolt) wheels have heavy outer shells, much beefier.
Several places sell the PVC inserts. Staz/TWF/ or you can find a piece of PVC pipe and cut your own. Here is a thread i started on pirate discussing light weight insert options, Andy at TWF did some testing and I did some math
http://www.pirate4x4.com/forum/showthread.php?t=1083795&highlight=
With the plate centers mouned as far in as you are wanting, I bet you could put some gussets on the inside to brace up the flat center. I'd think with as large as the bolt pattern is on a Rockwell, the centers wouldnt' be that bad to bend anyway.