Norvil Fork Slider

I think the disc size was limited by the AP Racing calipers of the time which were designed for rather smaller diameter discs fitted to cars. The PR disc (let's not call them Norvil) would fit inside these calipers, just clearing the body at each end. But the large disc did not come to the outside of the pads, leaving a small strip standing proud as they wore. The later AP calipers, the ones with the squared-off casting, will take a larger disc and should accomodate a 12" one - but do check if turning a couple of mil off the outside of your disc would upset you.

The Grimeca is, afaik, a straight knock-off of the later AP caliper.

A Norvil disc is 11.5'' though sometimes confused with the 12'' disc. If it is actually 12'' it can't be a Norvil disc. 

Les Emery's adapter bracket for AP Racing/Grimeca calipers comes (afaik) in different versions to suit different disc sizes. Mine, as the picture shows, is for the disc that the factory offered for roadgoing machines from 1972 onwards.

I've never liked the RGM version — the idea of slotted mounting points for brake calipers fails to reassure me.  

The PR-type slider from AN that you mention is presumably one of the versions offered for their 13.1906/7/9 brake improvement kits. These are intended for the PR disc set-up (adapted to the five-hole road hub). While I can't think of a reason to suppose that the lateral location of the PR disc varies from that of any other set-up it would be as well to make sure of this before spending money. 

I've measured mine: original fork leg from the '70s with a 11.5" Hemmings disc and a late type AP Racing caliper. I can easily insert a 4.5mm drill (but not 5mm) between the disc and caliper. The disc is spot on 11 1/2". If we call the clearance 3/16" that allows an extra 3/8" on the diameter, i.e. a permissible disc diameter of 11 7/8". Andover are pretty good a making stuff that's an exact match for the original so, if your disc is a full 12" then it's not going to fit.

If it were me, I'd go for the PR fork leg and skim 1/16" off the perimeter of the disc.

Agree with your choice but please don't just take my figures as-is.

I'd start by taking off 1/16", reducing the radius to 11 7/8". Then do a trial fit. Allow sufficient clearance between the disc and the caliper for expansion. I calculate that the circumference of the disc will expand by 3.8mm when heated to 400 C (hotter than you're ever likely to go), which equates to 0.6mm on the radius - MIG welding wire can be a useful gauge. And then check for any contact between disc and caliper in use.