Edited version of my comments above...after previewing I realized my grammar and spelling made some of my writing make little sense unless you guessed my meaning... In future I'll be sure to re-read or at least not post late at night when I'm really ready for sleep or more beer... anyway, this is what I meant to say, I think...
I did experiment with mounting caliper brackets inboard of the forks, ie making the mounting brackets the thickness of the nuts, as per the Lightnings. but decided against it for a number of reasons. 1.. mounting the caliper and speedo drive, I'd seen a caliper on one side and the speedo drive on the other (I don't doubt one caliper can be adequate for most people.. the forks don't twist, that you'd notice, really!) 2.. the caliper brackets needed milling, or bending to mount the calipers to align with discs. I find with any disc (conversion) caliper mount there needs (ideally) some amount of room for possible shims to achieve correct and proper alignment. 3.. the caliper bracket possibly may need to be of such thin material that lighter material such a aluminium or dural may not be of insufficient strength, the brackets cannot have an chance of shearing around mounting bolts etc. Milled thinner of bending to fit, reducing strength, is not at all good for a brake mount.
I agree with Davids 'generalized' suggestion for brake disc and caliper sizes, the Vincent twin and singles are relatively light bikes and don't need big brakes. Also consider tyre sizes into that equation, Vincents have narrow tyres and small contact areas on the tarmac, weight distribution under heavy braking also should be considered, personally I think Normans steering head mod with 'modern' suspension absolutely essential when combined with 'ultimate' braking capability. One last consideration, and not likely to be an issue for us, with spoke wheels, floating calipers are certainly better than those with opposed pistons (that are usually too wide for the spoke wheels) Floating calipers are less likely to warp fixed discs. Contrary to our hopes, hydraulic pressure on each piston in a caliper is not equal, due to friction and 'stiction' etc, in fact the Japanese 'built-in' for this effect with the ZX type calipers with different size pistons, I think these types of caliper work best for our needs...
Master cylinders... another big subject, I try to follow manufacturers specifications, I'd advise new or very carefully rebuilt.
Oh, and one last advantage, easy to include a front brake light switch, a hydraulic switch, either in line or on the master-cylinder banjo bolt... Be Safe Fellas...