Unbelievable, but then 5/16 is not very much for bolts to have a good load on the crank end. When finally success would you post photos ? Really these threads used for some trickery of lifting clutch plates ?? As said, the BMW wheel got M 10x1 mm fine thread but then it is a big 35mm taper, about 5-6 degrees per side. So maybe Douglas had less than that so in consequence disassembly may be a PITA ? When you look at the BMW crank, they had 35mm mainshafts front and rear, the sports S -types barrel roller bearing at the rear, 35 mm i.d. ball bearing at the front, only 50or 600 cc, but 7000 rpm for 42 hp R 69 S . The Vincent engines a mere 1 " mainshaft . . . .
Well, to keep to the topic, I think I´d go for next size threads for more crank end load and a good swing of a steel hammer again. Heat is no help here, same materials on both components, so no luck here.
Vic