personally i think the stock 4 piston calipers are miles above the single piston calipers in the heidts kit. also, if the heidts kit uses stock mustang-II brakes the rotors are only like 10" in dia., not sure if heidst uses stock M-II brakes or not, but you need to check and find out. honestly the only advantages i can see to the M-II kit would be more engine bay room, rack and pinion and for cars rhat came with drum brakes the kit has discs. the disadvantages outweigh the advantages though IMHO. the stock early stang suspension is a better handling/cornering suspension with a few minor mods than any aftermarket M-II suspension will ever be, in fact a stock M-II suspension is probably better than an aftermarket M-II kit, i personally don't like the fact that the aftermarket kits do away with the strut rods even if the lower control arm was converted to a true A-arm style setup. your choices for big disc brake kits are more limited and probably THE most important, the loss of structural integrity due to the loss of the shock towers. the shock towers are what holds the whole front end of the car together and gives it stregth. the M-II kits take away all that support and do not replace it with anything other than a stronger crossmember, but that stronger X-member does'nt do any good if it's mounted to thin sheetmetal "frame rails"