Are you just looking for disk brakes? Or are you wanting IFS and power steering, too? If you're just looking for the brakes, you don't need to subframe or do a chassis swap to get it. There's several conversion kits available that will put disk brakes on the stock I beam axle. Prices will vary, dpending on what is included in the kit. The economical way to do it would be to get the bracket kit for the calipers and master cylinder, then source out the master, calipers, and rotors from the flaps.
If you are wanting the IFS and power steering as well, you might look into using a complete crossmember form a '73-'87 C 10 pickup. That will keep the stock frame intact, as well as keeping a truck rated suspension and brakes. There have been several that have done that swap on the site.
