Sort through your bolts 'til you find one that looks close, try it in the hole and note if it fits. Most bolts will be std. sizes, 1/4-20, 5/16-18, 3/8-16, etc. for bolts with nuts, or could be the matching NF sizes of 28, 24 pitches. Get yourself a chart of NC and NF thread sizes to hang on your wall. In general, NC bolts are used in cast iron and Al, NF bolts are used in steel.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.