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.

The Baby Blue color is a very '70s color, just today someone said they love the color.  I've yearned for years for the '71 gold color but I've learned to love this blue one....like you say, it works!

seem to be having trouble with copy and paste of link.
sorry.

https://www.ebay.com/itm/296393712768
Saw this one...so nice. My first Pinto was a '71, exact same exterior and interior, but vinyl roof instead of a luggage rack.
If only I had another stall in the garage.
This one has a reserve so will watch to see what it sells at.
Oh well.  At my age I've learned to admire, without having to acquire.
I've been thinking: why am I so drawn to this eBay car? If it was another color, it wouldn't grab me so much.  It's the color!  My first Pinto was this color.  What is it about color that gets tied up in the emotions?

I think next week my Winter Storage barn returns to workshop status!  I will be starting back up on my project.

Unrelated, I have an 03 Ranger with a rotted bed.  There is rot around the wheel wells and the bed floor.  If there wasn't a bed liner in it, it wouldn't hold any cargo.  Today I just bought a Mazda bed from a guy that gets them from NC.  Rust free and the same color.  The body lines are slightly different, but it will bolt on.  I will clean up the underside, epoxy, enamel, then coat with fibrated roofing tar.  That will get me a few more years out of the truck.  While the old bed is off, I'll clean up and seal the frame with POR 15 or similar.  The left side wheel well is way worse than the right side in the pic.