I just went through this exercise, last fall, on my 1972 250. The bolts are 10MM, 1.5 pitch(I believe), grade 10.9, 2 of them are about 60MM long and the recessed one is about
25MM long. Check to see if the missing ones have broken off in the box. Also check to see if there is any hairline cracking in the frame rail, around the lowest steering box bolt. I had to install a 1/4 inch thick steel plate to reinforce my frame rail and prevent the flexing which repeatedly caused the steering box bolts to snap. Good luck.
Happy Motoring, Mark