There are a few ways to come at it.
1) Cheapest DIY way to do a "Shelby" drop is to get a template for about $60 to make sure new drilled holes are spaced correctly. May have to "massage" the shock towers a bit w/a hammer to allow clearance for the grease fittings once UCAs are lowered.
2) Can get some UCA's that have a dropped pivot shaft. They're bloody expensive, but no shock tower drilling required.
I did a UCA drop, since aftermarket arms were out of my budget. Glad I did it though, as I think it improved the (neg) camber curve. If I can find it, I'll post a little .gif I made of the front tire on the road.
**Points to remember**
--once you do this mod, a lot of alignment shops may not want to touch your car (liability issues?).
--May (probably SHOULD) want to get a bumpsteer corrector kit. Your front end will ride lower, and b'steer may be exaggerated a bit.
--Just make sure you get the correct template to suit your car! '71-'73 "Mustang" templates drop the mounting holes directly below the originals; early Mustang templates relocate mounting holes below AND slightly rearward of original mount location. 70-'73 Torino/Montego/Ranchero/Cougar suspension components/geometry are supposed to be the same as '71-'73 Mustang.