Aaron-

The technical term for these are "clutch head" screws and bolts. We have all had to find proper tools for these. I know that my local NAPA store sells a series of sized screwdrivers, still used for adjusting some carbs. Other tool companies sell clutch head inserts for ratchets and the like. Many suppliers for old trucks also carry these (I've seen them in the LMC catalog) Check with your local stores and ask for "clutch head" bits.