That 2.5 internal ratio is going to call for a pretty extreme spur/pinion combo.
Anyways, I don't know if it will fit for you or not, but you're looking at around a 57/80 pinion/spur in 64 pitch to hit right around a 3.5FDR.
Best I can suggest you do [as I did] is take your existing spur and pinion [assuming you've been running 27T brushed] that puts you somewhere in the middle of your motor mount adjustment, to your local hobbyshop and eyeball that pinion/spur combo I suggested and see if the centers line up reasonably close while meshed. If so, you "should" be able to fit it into your car, and still give you adjustment range up and down on your pinion.
That's again assuming that nobody else that could give you a proven working gear ratio for your TC3 chimes in.
Sorry I couldn't be more help.