ARGH - WD shank bolt diameter???

Hi all,

So, I can't use my current WD shank, because it just doesn't provide enough drop (only does 1.25" as near as I can tell). I have another shank, but frustratingly the holes are nominally smaller than 3/4" diameter, which is what my current bolt diameters are.

So clearly if I want to save myself $80-$100 bucks by NOT buying yet another shank, I need smaller bolts. As near as I can tell, I'd need 5.5" x 5/8" -- my concern is whether a grade 8 bolt of that diameter would be sufficiently strong? I think it would be, particularly since we're not talking tongue weights about 700lbs (I would think) -- I imagine the 3/4" bolts I have are 3/4" to accommodate the ~1200lbs tongue weight my system supports.

Anyway, any help would be greatly appreciated -- the other blasted issue is where to buy such a beast -- so far, the only local entity would be Napa, which could order 5.5" x 5/8" grade 8... grr...

Thanks,
John