Installing seat belts....Which holes do I use?

[aod92]

I'm about to put seat belts in my 67 c10. I have non retractable lap belts. Behind my bench seat there are 3 holes on each side. Do I use the hole that's furthest to the door, and also the one that's furthest to the middle of the cab? Also do you guys know the bolt size?

[72farmer]

I too am trying to put seat belts in my 67.I took a set out of my 72 GMC donor truck.It had 1/2" course thread bolts.After trying to run a 1/2" tap into my bolt holes and ruining one I found out they were fine threaded 7/16X20.I don't know when they changed from 7/16" to 1/2".