Nice build.
So you're building a new frame for interior clearance, etc?
The Mustang II front end idea struck me as questionable off-the-cuff but after a minute of thinking about it, it makes good sense. I can't imagine the car needs very much front wheel travel, riding as low as it does. And the fiberglass body probably doesn't bring enough curb weight to press the M2's structural limits either. A 141-in wheelbase Chevy Impala sounds like a huge car but it's more dimensionally huge than massive weight.
I'm assuming the axle housing is only tacked to the frame in these pics for mockup purposes?
Do you have much trouble with the body's front overhang & low height clearance? That always struck me as such a potential PITA for the '89 Batmobile. Every time I see a streetable replica I get the impression it's sitting a little bit high in the front too.