bigger rear disc in rear than front a problem I assume?

1WildRT said
.26 in basically a quarter of an inch, and that's track width so for both wheels... That means one eighth of an inch per side.... If one eighth of an inch causes a issue with tire/wheel fitment than they were to damn close before the swap...

That’s not how Wilwood specs it. It’s listed as “hub offset”, not track width. It’s per hub, so that’s .52” to the track width.

newguy said
One kit I'm considering is 140-14271, that hub is listed as 2.76" and I could not get a caliper inside the cop wheel to measure, but measured the hubs on my 74 disc brakes and that measurez 2.8", so it will fit BUT it states it increases the track width .26", so I'll need to mount the wheels again and see if I have that much room to clear inside the fender

It’s compared to the Mustang hub. So unless you’re already running the HDK with factory Mustang II hubs putting the wheels back on won’t help you.

You’d have to add .52” to the track width of the HDK and then compare that to your current track width.