Do you have any info on when it was built? DMV records should show when first registered. Unless it was a DMV assigned VIN, the VIN may provide some here with a clue. Check around on the frame for some numbers also.
Is the wheelbase 90 inches?
Just some observations: It has kind of a mixture of 289 and 427 features. The forward braced roll bar and sidepipes coming UNDER the body are like a 289 instead of through cutouts like a 427 (although the 4 into 1 arrangement is like a 427). Kind of hard to tell exactly from the pictures, but the front wheel well flares look like they may be coming out a few inches down the fender like a 289 instead of at the top of the fender like a 427. May just be the picture angle though.
But then there are 427 specific features as well. The oval front grill opening (289 is flat on bottom), rear flares tapering into the body at the sidepipe exits (no taper on the 289), and fuel fill on the fender indicate 427.
I point these out in case someone knows of a manufacturer who combined features from each. Some other notes…The steering wheel has round openings instead of slots. Radiator and positioning, from a Mustang perhaps? Power steering. Louver positioning in close proximity to flare. Hood hinges.
This may help as well:
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