I dont think the GT driveshaft will mate up to the T-5 ... From what I hear in order to mount the 8.8 to the V6 you need a cobra pinion flange and you can keep your driveshaft
Its actually more cost effective to replace the 8.8 than it is to build up your 7.5 to be honest, especially if you already have the desired gears in the axle than you can pretty much do it yourself.