Hey Josh, the 3.5 is nuts! Glad your running a 6.5 as that'll actually be quicker over a lap on any indoor track than the 3.5..
On the GTB, I run mine with factory default settings but turn down the drag brake, in 2wd to the 2nd setting and in 4wd to the 1st setting (= zero drag brake). So then all you need to do is make sure your throttle curve is linear on the transmitter and however you hold your tranny make sure you can control the proportion of the trigger
im sure you'll be fine.