go to junk yard buy spindles and rotor and caliper off a 95 or 94 then go to ford dealer and ask for 5 lug drum convertion for the rear about 225 for rear and 150 for front
dude to be honest if your gonna do the job you mine as well make sure you have ALL the right parts, I know you can buy a complete kit from 50resto for 400$ and that includes everything. I did mine.
I would go the 'junkyard' route, the resto kit limits you to wheels off 94-95 gt's stock and a few other choices, but you would probably have more choices if u just stayed with 4 lug. I think if your going through all the trouble of changing everything, might as well get disc's in the rear as well. Try to find an entire set-up from a 94-95 mustang (easier said than done most of the time)
I personally don't see what the big deal is about 5-lug anyway. A lot of rims will fit the stock design not to mention there are a good deal that are made in a 5-lug conversion as well! Pick your rims you want first then decide if the 50resto kit/stock spindles with the large hub design will work and second if they are even 5 lug in the first place!