The only thing you need to buy from Heidts is the crossmember, top hats, and strut rod mounts....generally known as a crossmember kit. You don't need to buy all their other stuff. Stock or dropped Mustang II spindles are available almost everywhere and the Speedway kit for the big GM brakes will fit the stock Mustang spindles. Just keep in mind that you will probably have to go with 15" wheels with those big brakes. Shop around for the best deal on control arms or stop by your local wrecking yard to see if they have any used control arms. Chances are they will be good factory Ford parts and all you'll have to do is pick up some new bushings and ball joints, clean them up and hit them with a coat of POR or your choice of paint. Probably be able to get the strut rods there too all cheaper than the aftermarket.
"Instructions are just the manufacturers opinion of how it should be put together". - Tim Allen