Wow! That doesn't rub on the fender?
Well, this is the info you need here, post #6
Measuring for Backspace Front Wheels
Keith Mopar has 17x8's with 225/45/17's on his '65 cuda with 5.5" of backspace. Only catch is he says he has wilwood disks, at least in the rear. Not sure in the front. If his track width is the same, it would mean you could run 17x8's with a 5.25" backspace and 245/40/17's up front. Same clearance to the fender as your current set up, same clearance to the suspension as his. Problem is I think his track width is probably wider with the wilwoods than yours is with the KH's, I'd have to know what kit he's using.
Thing is you should be able to run similar backspaces as the later cars, after all the control arms are the same length. The KH disks have a narrower track width, so that would reduce the backspace some compared to the later cars with BBP disks. The sway bar set up in the front might limit it too if you have a stock bar. Not sure if they have sway bars for the early A's that are shaped for big tires.
Like I said before, Early A's are not my area of expertise, I'm just comparing other set ups and looking at the math. Obviously there's more to it than that, so hopefully someone else will chime in.