Great mod, wish I could weld..

I did something very similar in my '67 coupe as I'm 6'1" and added '90 fox body seats for shoulder/neck support.

I used the '90 seat tracks and drilled a flat bar as you but used grade 6 (highest I could get) carriage bolts and filed square holes so the slides would clear. I did not cut my floor pans or drill new holes and used the stock mounting points in the seat bottoms for strength.

I cut my flat bar at 14" and it gave me more rear slide space than I even need and the '90 seats put my legs no closer to the wheel as they appear the same height at the knee bolster.

Next step is to get seat covers to fit the '90 seats and match my rear seats horizontal factory stitching and 3 point seat belts.
I can post pics in a new post if anyone's interested in this.
Jon

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'67 Mustang Coupe 289 AT modding Trans Am style
"If it ain't broke, I haven't fixed it yet"