Its not cynicism but realism.
I have done some marathon rides on vintage bikes, and I mean vintage; the best I ever managed in daylight was around 400 miles on 1938 500 in France with relatively traffic free roads.
Not bad weather mind, a nice May day,not in the hours of darkness and at the end I was totally knackered.
you are making the classic modern rider mistake of backprojecting your wishful thinking re speeds and possibilites.
There is no way you could ride a 350 Matchless, which was the best and fastest of the WD machines save maybe a good Ariel, at 60mph all that way in the hours of darkness even part way ,
I suggest you scrounge a ride behind a blackout mask one day........
At 60mph its near flat out. Brakes are, well try one for your self, they are 5inch diameter......
He would also have needd to refuel at least twice, where? with rationed petrol? at an Army base of his route? not likely
That is a nice Matchless but its still not a "scrambler", whereever did you get that notion? I repeat Matchless did not make a 750.
I thin its a good project and wish you well, but I still think you need to sit back a bit and do more research.
No disrespect to the old feller, but memories play tricks.