FENDERWELL HEADERS/ FRONT RIM SIZE

Doing something different with my 64 Dart, gotta 636 ive just stated setting up doing semi fender wells, 2 inner pipes through inner fenders and other 2 inside, also did a few other mods to rails so it will have tire clearance and wont have to sit up like a 4WD

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Just wondering - is it possible to do some mods to the firewall in order to get the motor back enough to run a 26" tire? Might not be possible, not sure what is keeping the motor from going back an inch or 2.