I've recently gotten back into scale model building... well, more like I've made a list and started collecting some of the kits I want to build, that is (there are a LOT, I must say).
I've found several '71-'73 Mach 1 & Boss 351 kits, in both die-cast metal and plastic. Unfortunately, I don't believe there are any coupe or 'vert kits out there.
My intension is to get enough kits to build several versions of my car (from the factory, "rusty pile" as I purchased it, restoration in-progress diorama, and as it sits now), along with some other versions... one of them being a 'vert. I think building a 'vert would be the easiest based on the fastback kits out there. It'll take a lot of work and fabrication... but cutting off the roof, trimming down the trunk & quarters, and fabricating a new rear deck shouldn't make too much trouble.
Once the rear deck/trunk is established, make a coupe should be possible... getting the roof, quarter windows, rear glass, and 'buttresses' to look right would be the tough part - I don't know if it would be worth it to attempt to get the taller roofline correct, but it might look weird if it's not done.
All of this is still a long way off for me - I have WAY too many things going on in my spare time right now to sit down and burn so much time working on model kits. Once I get my garage, shop, and storage situation taken care of, I'll have a permanent place set aside for this (an airbrush station and scale model "paint booth," if you will), and would hopefully start burning through the kits shortly after.
I've been checking on ebay. There are plenty of kits out there - some of the more rare and detailed kits can get expensive (I paid over $100 for a new-in-the-box Japanese 1/12th 1971 Mach 1 with a 1/25th scale version included - Doyusha Vanishing Mach 1... which I recently have seen as high as $250). There are also people selling pieces, parts, partial kits, entire lots of leftover parts, wheels, engines, etc., along with completed 'glue-bomb' kits... which can be salvaged and made to look nice as well - perfect candidates for the extensive modifications needed for a coupe and/or 'vert.
Sorry for the novel - I just really love scale models. It was the only way I could afford having a cool car (or a car in general) back in the day when I was a kid.