a 68 GTX should be the same as a 68 RR,
unless it has a 4 speed & Dana-60, it's a lil' longer, like 1/4" (?) IIRC 'each side'
if you have disc brakes it can push the wheels out another 1/4" on each side too
I think my 68 Charger R/T had/has a slightly bigger wheel well opening
my 68 RR
it's like 13"+ (give or take 1/4")
between the trim to the edge on the inside of the wheel tubs
there's also a big hump on the inside of the outer part of the inner wheel wells
that could cause rubbing on extreme angles, tire going up into it
so for no rubbing ever
on the rear of my 68 RR stock height, clear no problem
BFG Radial TAs P295/50/15 on 15x9+ Halibrands
(mounting surface is like 8"+, really thick edges for the beads)
w/5"+ BS
has like an 1" clearance on the trim/outside & the leafs/inside
still sits too high in front, need to lower the TBs & get realigned
BFG Radial TAs P245/60/15 on 15x7" w/3.75" (or 4" ?) BS IIRC