Hey,

Yes, the thinner bolts are for the rear mounts.

Sounds like you have the front shims in the right place. I have the rubber pad on top of the shims. It is an anti squeak device I think it will do more good on top against the floor.

The rectangular pad goes on top of the mounting bracket with the round pointing down, it fits in the hole in the mounting bracket.

Yes on the spacer pipe.

The factory bolts did come up from below and into nuts that were permanently mounted on the floor inside the cab. After 50+ years who knows what work has been done to your truck. When I rebuilt my cab I had to replace the cab supports and that eliminated the factory nuts so I now mount my cab with bolt head inside the cab and the nut below.

Keep us informed of the progress