Just build one yourselves... it's not all that complicated. There are benefits to using rear mount systems - mainly the ability to go back to stock rather quickly and keeping the engine bay clear. You do have to solve logistical issues of the oil return (140$ shurflow pumps work great) and the minor fab work to get everything mounted. On a 6psi kit, you wouldn't even need a front mount, although you could if you wanted to.
I don't like STS because they rebadge cheaper gear as being "special" with a hugely inflated price tag. Seriously -- spend 2K and build yourself a far superior setup than having them build one.