oops... i didn't scroll down far enough... I should have realized your description didn't macth the first couple of front ends...
Anyway...If you go with a MII front end... here's the one I was thinking of in Street Rodder magazine. A few years ago "Helix" seemed to have some pretty aggressive introductory pricing. Here's one for your truck on eBay... $1458 delivered...
http://cgi.ebay.com/ebaymotors/55-59-CHEVY-PICKUP-TRUCK-MUSTANG-2-II-IFS-FRONT-END-/190548748403?pt=Motors_Car_Truck_Parts_Accessories&hash=item2c5d972873
This is one of those apples and oranges comparison... the MII is all inclusive and relatively easier to install. The kimbrige require a huge commitment to cut up your existing frame and hope that you get the new frame aligned right. It doesn't seem hard from their instructions, but one screw up can have a big impact, not to mention there is no turning back once you cut the frame. Once the kimbrige is installed I'd venture to guess it would be a bit more stable considering how the lower arms are mounted. I do like that the lower control arms are mounted in a manner that spreads the load some onto the frame rails as compared to traditional tubular MII setups that concentrate the load strictly on the cross member mounts. Most folks would prefer that the control arm mounts are configured in double shear, but most MII setups use single shear mounts. Lots of folks say it's not a big deal, but from a design perspective double shear is better than single shear.
If it makes you feel any better cost wise, I built my own front clip for my Mustang because I had all the ancillary Fox based pieces to do so... but after I got started I kept replacing parts with better pieces and ended up spending over $1700 plus raw stock cost... I can't tell you how many hours went into building the clip, but it took quite a bit of work to get it done...
If I get another Mustang, I'm pretty sure I'd go with a rod and custom front end and possibly add some bracing to better support the lower arms.
Either way you go seems like a bargain, but the MII would definitely be an easier installation to do at home. You could essentially tack it in just to get a feel for how it will work before committing to welding it in... plus you don't have to mess with the potential body alignment issues.
The kimbrige could also be fairly easy to do, but could also be very easy to screw up bad... once you cut you are committed... there is no going back!
I think the only downside to the MII setup is the single shear lower control arm mounts... check around to see if any MII kits are available that mount the lowers in double shear... it might not even be an issue... just something to consider.