The MM torque arm kit does seem to be the best bang for the buck. Eliminating the upper control arms will drastically lower the rear roll centre, so the outside front tire won't be overloaded as much (in a turn).
[plus you don't get the aweful rear jacking when braking].
The inside front tire looses grip long before the outside does... so using some heavier springs will really help keep it down.
Some front urethane control arm bushings will help maintain the crappy alignment. Throw on a lower chassis brace, and you won't believe the turn in response.
Some good subframe connectors are a MUST on the convertible. The really help the car transition from one turn to another... actually letting the suspension do the work rather than twisting the floor pan.
Going with the MM stuff up front as well is a good budget fix... get some CC plates, and go for MAX caster, about -1 deg camber, and 1/16" toe in.
You might wanna check out some of the stuff (CC plates, control arms, etc) at
http://www.hotpart.com/ ...AWESOME prices.
www.ipschassis.com has some interesting stuff (good info on mustang suspensions too). The Sport front gives you an SLA setup for $600 (using stock LCA, springs, etc). When you consider the fact you won't need cc plates or a tower brace (since you eliminate the struts), it's not really that much. Lots of caster, camber gain, anti-dive... I don't know why more people aren't jumping all over it! I think the rear 4 link is a better idea than a torque arm, but the modified Watts link lateral location device is just to complex... I'd just go for a simple panhard bar.
Grigg's is finally developing an SLA front end, which should be totally awesome. It'd be nice to actually have camber GAIN in a corner. It'll probably shed about 75# off the front, but will probably run $2K or so for the K-member, coil-overs, etc. Pricie, but it'll probably be the ultimate setup.