Any idea how many threads you will have left after machining (if you go that route)?
I was taught that much past a bolt diameter and a half is unnecessary in Iron.
So if they are 1/4" bolts then 3/8 should be good (1/4 X 1.5 = 3/8).
I would even try 1 shank diameter, if that made the difference (especially since that particular pressure plate shouldn't be especially high pressure).
At least I would if that flywheel aligns the cover with dowl pins.
Good luck.