The first answer would be MONEY. Since you got no replies I took a cursory view. Kits seem to range from about $800-$2,500. In a post on another Forum someone mentioned that the earlier spindles were smaller. Oddly the kits show 71-80 Pinto so maybe they include a new spindle??? Be aware that a lot of the kits upgrade to the 5 bolt pattern. Also a lot of these kits are for drag car brakes that are thin rotors and likely not conducive to everyday driving. Both are something to be on the look for.

My personal recollection is that there are Ford factory parts that the swap can be done with. But off hand I don't have a list. I also remember people saying the whole (welded in) cross member had to be swapped??? Everyone wants the early, thin bumper Pinto, but there was a story on this site where someone had a later 74 Pinto and they swapped over the earlier body parts and it seemed easier than working around the brake/steering rack issues of an early Pinto. Sorry I can't be more help.

If you are going to weld in a 74-80 crossmember, you may want to consider using a Speedway or Heidt's crossmember.  Their aftermarket parts will bolt up to their crossmembers with less modification and hassle than using the stock 74-80 crossmember.  I had some issues with Speedway parts with my stock 80 crossmember.