Independent rear suspension (IRS) shouldn't be that difficult for someone with reasonable fabrication skills.
C4 Corvette might be the easiest with the least amount of hacking and can be had for under 500 bucks. Trailing arm brackets mounted where the front spring hanger mount should be doable and mounts applied to the stock longitudinals for the crossmembers sound workable too.
Cobra Mustang stuff sounds expensive because of it's rarity, has a tendency to shred axles, needs aftermarket upgrades to address geometry issues, and would most likely require hacking out the floors from the rear footwells back, and back halving from the front longitudinals.
Although they would probably require the same amount of hacking as the Cobra stuff, the MN 12 T-bird (89 up) and Mark VIII Lincoln are far more affordable (I've seen the standard T-bird stuff go for as little as 125 bucks around here), use a fairly strong 8.8 differential, the Lincoln and Super Coupe T-bird even use the same aluminum case as the Cobra, the stock control arms pivot off the subframe and don't require trailing arms. The subframe looks to be adaptable (welded to) to sub rails connecting the front of the car to the back bumper, and carries the suspension pick up points, simplifying assembly.
And although I haven't done much more than superficially look at it, the Lexus IRS might also be reasonable option, size wise, if you're not running a torque monster.
You could always buy a wrecked Viper for a donor, hack the entire bottom out of the car, lengthen the Viper chassis, and drop the car around it, with or without the V10.
It's quite possible, even with all the work required to convert to IRS, even by a builder extremely knowledgeable in suspension design theory, you would find a well sorted out solid axle car staying with and even passing the IRS car.