I just put a new pioneer deck in my 60/ the dash off. Fiirst I riveted to sheet metal wings onto the sides of the new deck sleeve so that its profile was the same as the stock radio plastic trim. Hammered the rivets flush so that the deck could slide into sleeve. Painted face of sleeve...
Being that it was a metric bolt, I used a 45/64 bit in the drill press,but it was a tad tight .Had to torque the the head around some with the bit in the hole. 23/32 might have been better or too loose -dont know. i think I pre-drilled with a 5/8 to center the shackle in the vise. I re-used...
I assumed they were just regular steel . I drilled my stock shackles to accept greasable bolts and they cut easily . You could do a blacksmith's spark test on a grinding wheel- compare sparks given off by the shackle with a regular bolt and a hardened pc. of steel.