Hi Glen.
I did the same kind of research last year as I was building my car for road racing and vintage rallies. The first I saw was Magnum Force Racing's. I didn't like it because (back then) it had no engine mounts (elephant ears were mandatory). And it used the inner fenders for coil-over support. I don't know how you feel about using 1.2mm-thin sheetmetal to dynamically support the 950 kilograms that are resting on the front-end, but 'street-worthy' isn't what comes to my mind.
I ended up using Reilly Motorports' AlterKtion setup for two main reasons :
(1) Up until a short while ago, Bill Reilly's K-member was the only one to have an integral upper shock mount, whereas Magnum Force, CAP Auto, and others use the OEM fenderwell mounting point, which is not meant to absorb the thrust of a coil-over shock.
(2) Of all the guys I asked information from or had prior business with (CAP, Magnum Force), Bill is the friendliest, most helpful, and his setup has a lot of supporters (on moparts and other forums).
Bill has started making torsion-bar versions of his AlterKtion too (with short torsion bars hidden inside the K-member tubing).
Now, there are two other suppliers for K-members that feature complete suspension mounts : XV motorsports (extremely expensive), and Control Freak Suspensions (an AlterKtion knock-off, the firm primarily makes Ford & Chevy suspension parts, they started on mopars this year). I wouldn't recommend either.
Installation is easy, the only real work is to remove the bump stops from the fenderwells and drill 2 holes per side on the framerail.
A friend of mine made a few videos when we installed the K-member in my car, here they are :
http://www.agitatum.fr/video/4hb/index.html
I've used very stiff springs, and I'll go back to lighter springs this winter (550lbs/inch to 450).
Other modifications include Hydroboost brake assist, 13" rotors and 6-piston calipers (Wilwood setup, the K-member uses Mustang II spindles so brake kits are easy to find).
I'm so satisfied with the front-end that I'm installing Bill Reilly's rear 'Streetlynx' setup this winter.
My car handles like a sports car now. The rack steering is good, assistance level is fine (not too assisted).
I do recommend frame connectors and torque boxes, though. I'm installing these this winter too (well, If I can get a hold of Auto Rust Technicians, that is).
Now, I don't know what you plan to do with your car, but great improvements can also be found with the original K-frame, big torsion bars, good shocks, etc. for much less (less assisted than stock steering box from Firm Feel, 1.12" torsionbar, swaybars front and rear, double adjustable front shocks). DrDiff makes adapters that will let you mount a 73+ brake rotor (including the big 11.75" cop-car pieces) to a 1970 spindle.
There's another guy on this forum, from Poland, who also installed an AlterKtion, maybe he'll chime in.
You should definitely head over to Moparts and look around, too. Lots of people there with experience in tubular K-members.