i would properly convert the car the the large bolt pattern front disk brakes you could do for less than $400 and get a nice set of lbp ralleys for cheap, since sbp ralleys are super expensive. this will forever put an end to the sbp lug pattern on your car as well as give you much better brakes. i dont have much faith in an adapter of any kind that holds a wheel on.
also if your worried about have in 2 different bolt patterns, its easy to change the rear axles, or if you want to cheap out, you can have the axle and drums re-drilled to be lbp, but i would just change the axles or get from a later model car. seeing this is a /6 forum, i imagine your car has the 7.25 rear, shouldnt be hard to get a complete 73-6 abody rear with lbp and bigger brakes for around $50 or less. ususally people give them away. i just junked one.
if you plan on doing the work your self, it wont cost that much and will be a huge improvement, however if you pay someone to do it, it'll prob cost alot.
parts you will need to get to convert to disk brakes are: basically everything off a disk brake donor car, so you'll need for both sides: upper control arm, knucke assembley(spindle,rotor,caliperbracket), caliper, brake hose, and lower ball joint(which i would replace), in addition you will also need a disk brake master cylender and proportioning valve.
i like to pull the whole thing as a unit as seen below in the pic. i find it easy to do,but you have to have the right tools.
anyways, good luck with the car hope this helps.