Some older engines, dating back to the Moggy, had shaped, hardened washers on the bolts which would lie in the groove properly and spread the load to help avoid distorting the cover. I assume they disappeared due to cost cutting, as my later Minis had plain washers under the lock washers. Definitely not as good, and the joint tended to weep slightly at the bottom.
You could make them from largish standard washers by grinding away the two sides. Best to start with through hardened washers and keep them cool while grinding, so they remain hard. Or get a piece of tough steel like gauge plate, maybe 1.5mm thick, and cut them from that. Or use somewhat thicker mild steel and slightly longer bolts. Two sizes are needed, and if I recall correctly they are 1/4" and 5/16" clearance bore sizes. Maybe Moggy and A35 specialists still have them.
They can be retro-fitted but I think would not be easy to find. There was also a stiffner which spanned at least the bottom 3 bolts, part 12A666.
You may well know this already, but if not, please be very sure to centre the seal. All you do is fit the cover with all bolts loose, so it can be wiggled easily, pop in the pulley, with its sealing boss well oiled, optionally turn the engine a couple of times, and then nip up the bolts. That helps the seal to have a long and successful life.
