Let's see.. Ok, the ford racing book has a 10.0787" deck height. The 5.4 has the same height, so there is your comparision for width, but no known length dim. Having supported many sbf first gen swaps, I do know that you will have a problem with your shock towers,because the ohc motors are substanbtially wider, but if your other choice is a 429 motor, it may only be an inch or so wider. The length, I think, will be the dealbreaker in that chassis, more than anything, as far as fit. That said, we go on to the issue of electronically making it work. You are looking at paying a shop like ours about a grand to convert your existing donor harness, AND, you have to import ALL of the security system components into the car, and each vehicle's system is VIN specific, meaning no getting some from that truck, some from this truck, and the PATS security system module IS the instrument cluster. Identifying the PATS equipped vehicles is by checking the igntion key. If it is the FAT-headed, 100.00 key, it does, if it has the home depot, thin key, it does not. May I suggest running a 5.0 or 5.8 EFI, with a custom fabbed induction, for the looks? Being different costs exponentially more than working with simplicity, fer sure.
Found the 4.6 specs, for your perusal.
The most popular conversion we do here is the 4.6 SOHC, but have not done so in the 1st gen mustang. Break our your tape measure.