I would have them chek real good for rust in the usual places.
1. lower cowls
2. cab corners
3. radiator support under headights
4. drip rail/rain gutter (check from inside)
5. floor pans between kick panel and cab bolts.
6. front cab supports

These are real bad about rusting.

 

Most industrail supply houses will have 7/16 bolts.
I use the big yellow catalog alot, McMaster-Carr Theve got everything and it only a day a way if ordered by noon i get it the next day. It's not OEM or NOS and 7/16 are never cheap but they have it.

44Dwarf

 

In addition to the spots Krosati mentioned, have a look at (look at it with a hammer!) the cross sill under the cab. After I fixed mine I drilled a 1" hole at each end to allow better drainage. You'll see where I mean. Also, there is a cavity behind the kick panel that is inaccessible unless the kick panel is removed. If there's no rust in there you don't have to worry. If the kick panel is rusted through, or there is rust at the seam between it and the floor, you might want to look in there.
I also drilled 1" holes at the bottom of the cavity under the vent door. These can be plugged with vinyl plugs. And - don't take this the wrong way - after you've sand-blasted the bottom half of the cab, inside and out, the trouble spots will be easier to see.
Eric D