How to set the cab straight?

[Gregski]

I have a '71 4x4 cab on a '69 2WD short bed frame (me thinks, ha ha) and everything seems to be polywhompus on this truck, which don't get me wrong I absolutely love by the way, the truck not the ... you know what I mean

So the fact that the bed sticks out about 1" further at the rear of the cab on one side let me to investigate a bit. You know how it goes, what's crooked? the bed or the cab type of thing?

well I wear glasses and some would say I'm blind as a bat, but I've always had an eye for unevenness and I know what you are thinking, whoa you can tell a 50 year old truck is uneven, genius! no not only can I tell it's off but I can guestimate which way it is off and by roughly how much

so the cab seems to be about 1/2 inch off from the front frame horns, but I think the truck got a smack on the front at some point so I think the horns can't be trusted, since I never could get all the four front bumper bolts into them horns, (there was always one odd man/bolt out) ha ha

so what's the best way to get that cab on there straight, what holes, or land marks would you go off of, not looking for perfect, just good enuff cause a crookend cab means a crooked grill etc, etc