Yes I couldn't agree with you more! Disc brakes are always a plus in any case with any car, aftermarket 4 lug wheels are available but not many to choose from. I met a guy back a couple months ago at a swap meetthat had some 5 lug rear axles from a '66 mustang 7.5 rear asking 75 bucks for the pair.
I told him I was converting to 5 lug wheels, I went on to tell that I was also coverting to a built 289 expecting around 300+ Hp
he advised me to go ahead and try to fine either a 8 or 9" complete rear end.
I'll see can I find his tel# but of course you would have to get all the brake assembly to change over, I beleave if you hold out you may find what you need in a salvage yard and simply score the whole rear end
as I have done.
I got mine from a '79 Granada although is a little wider, If you can find a granada that still has the front assembly be sure to get this as well it will work for your front conversion, I was not so lucky the front components were already taken, so I woundup getting what I need from a 64 Fairlane had drum brakes, But atleast I have the correct spindles when I do get ready to convert to disc brakes...
and keep a check on ebag but they can get pretty pricey on there.

 

I found this one up on now good price.
eBay Motors: 65 67 69 70 73 MUSTANG FAIRLANE DISC BRAKE SPINDLES (item 290194846690 end time Jan-06-08 09:02:34 PST)