You really need to look at the cars in-the-flesh and talk to the owner-builders: they all have quirks. A lot of your choice may depend on what you want to do with the car.
There are plenty of FFR's to see in MA, probably you can find at least one of every other manufacturers examples. The best is if you can find one under construction, or body-off, or at least on a lift, to see what's under the skin. I have a friend who,as a professional builder, has built several kinds of kits: now FFR is the ONLY one he will build. The local Shelby dealer even has to redo a number of things on Shelby's own "Continuation Series" before they sell them.
One problem with the other IRS kits is the Jag/Salisbury rear IRS: the Ford T-bird stuff is much better and much cheaper. Many kits were designed so many years ago that the Jag stuff was all that was available, and they've never been updated. Many use the front suspension from a Mustang II/Pinto chassis (actually, fairly common in hotrods, but I'm not too impresssed with it).
To me, the only real alternative to FFR would be Superformance: a modern-design, Ford parts, well-made car, big user base. If you want a daily driver with roof and defrosters, it's an only slightly more costly alternative. Biggest downside: you don't get to build it yourself.
If you're going FFR with a 351, Tremec, IRS (very good choices, BTW, although for roadracing a 302 is probably better <lighter>), the way to go is to get a "pallet donor", which is basically everything you need in a box: T-bird IRS, driveshaft, front brakes/spindles/steering rack, pedal box, brackets, bolts, etc. You could buy this stuff all new, but it would cost a lot more, since much of it has to come from a Ford dealer (or you could go with Wilwood pedals, etc), or piece it together from salvage yards IF you have a lot of free time. You can order a pallet from many sources that will have just about everything you need at reasonable cost, all guaranteed to work.
Take your time deciding: with any luck, you'll be driving the beast for a lot of years!
HTH
Forrest