About the time I picked up this 1937 Chevy truck for a project I also bought a '74 half-ton 4x4 for a couple-hundred bucks just to part out...the '74 had these trippy six-lug chrome-reverse wheels on the front, I stuck 'em on the '37 which still had it's original running gear just to see, and was hooked. I really did like keeping the six-lug. However in no amount of looking did I find any other six-lug wheels I cared for and certainly no old genuine chrome-reverse, too bad the beater 4x4 only had 'em on one end. So I picked up some 1960's regular steel ones for the rear, planning on widening them 2-3" for bigger tires and to have at-least the same amount of "dish" as the fronts, maybe chrome/maybe not.
The main point of the above is to re-iterate, there is not much out there for choices in a six-lug wheel, either you have original steel or the very modern (and often pricey) cast/billet styles, in my case I didn't find much in-between. You might come across a set of old slot mags somewhere, those came in six-lug versions back-in-the day.
The bigger problem is going to be your Mustang II front end. Those tiny little Mustang rotors made for 13" wheels (not saying they don't work, just that they're small) have about a 6.0" diameter face, the flat part the wheel bolts onto. The six-lug wheels have a 5.5" bolt circle, barely any smaller than the rotor face, add the half-inch stud diameter and you're right on the edge...if you were to try to fit the lug pattern you'd have to drill and tap holes (again, right at the edge of the rotor) and use screw-in studs (all thread, grade 8, I think Moroso sells 'em) 'cause there'd be no installing anything from the rear as normal. Then steel wheels are almost out of the question being as the stamped shape around the lug holes requires flat surface of just a bit over 7" diameter to seat properly. I say "almost" because in theory you could fit a 1/4"-thick 7" plate between the rotor and wheel to give the wheel the surface it needs but none of that is something I'd actually do, it's just getting 'way too creepy.
The 10 1/2" GM rotors which can be fit to some Mustang II kits still only have about 6 1/4" of flat face, well shy of the 7" you'd need.
If you really wanted to use six-lug wheels on a more modern independent front suspension, the way I'd go would be to use the 2nd-gen F-body subframe with swapped-on Caprice 12" spindles/rotors, which would have enough front face to drill the 6-on-5 1/2" pattern...a '70s truck front end as mentioned above would work too but you might not want the width they have in an older narrower truck.
(Me, I'm going to use '88 GM 2500 "light-duty" 11 1/2" six-lug rotors on the original axle, but that's just my choice and there'll be some machining to do for that.)
...But then you are wanting to still go with the Mustang II kit (which in my experience work pretty well although the costs add up)?
Swap on over to five lugs, is what I'd recommend. The MII rotors are available in GM and Ford patterns and the rear can be re-drilled, there's plenty of room on the axle flange. Your wheel choice goes 'way up, and as used wheels are far more available the costs can go 'way down if you choose, and if you don't know what you want just now, stick any old roller under it for temporary just to get the mechanicals in order first.