getting rid of the 7.5 rear and putting at 2011-14 8.8 rear in. Question I have is for rear springs, will GT springs fit or do I have to go with V6 springs still?
Get aftermarket control arms and aftermarket springs. The Ford Racing springs are nice ones. Make sure you have the cobra pinion flange to bolt up your driveshaft.
The 2011-14 rear ends already have the flat 'cobra' flange needed by a v6 driveshaft; makes that whole flange-swap a non-issue!
If you're on a budget, just swap over all your v6 suspension to the replacement. Otherwise upgrade as you see fit. Your rear springs are the same as on a GT; the shocks are a little different.