Oh boy. No you don't need a shim - they are "not required" and only used to level out a truck with a damaged frame.
Have you tried to release the through-bolts yet?
I don't mean to be a Dr. Gloom but, I would attempt to pull and replace those bolts before you begin this project. On trucks as old as these, the bolt and the "nut" (see the part outlined in red below) have often turned into one unified seized/rusty component.
You must be registered for see images attach
Your truck's mounting hardware may not be this way - but lots and lots of them are. What happens is, you buy your parts and get your tools out - all the time thinking that this will be a quick Saturday afternoon repair.
But when you go to remove the bolt, one of three things will usually happen:
1. The bolt does come out and you raise the cab a bit, slip in your new rubbers and drop the cab back down. You thread the bolt back in and drink beer. This is extremely rare.
2. The bolt does not easily release. Then while you're swearing and pulling on a 3 foot breaker bar - the bar lives up to it's name and breaks the bolt off. Then you are looking at hours of drilling to get the broken off stub out of the nut. Then you get to tap the hole in the nut out to the next biggest bolt size and source a new bolt. That's one option, you can also just drill it out and drop a new nut on top of the old nut. This is done from in from the inside of the cab (see the third scenario)
3. The bolt appears to easily break free and begins to turn for you. But, after cranking on it for a few minutes, the initial feeling that "everything is going great" begins to fade.
Next you start to get the feeling that something just ain't right. That feeling is quickly confirmed when an investigation reveals that the bolt and nut are still one, and you have been turning them both together.
It slowly dawns on you that what you have is; a nut that is free to rotate and you have no way to stop it. Now it's time to tear out the seat and begin cutting holes in the false floor where it rises up to the back wall. This is in an effort to locate the unsecured nut. So you find it, and now you have to find a way to hold it still while someone else gets under the truck and cranks on the bolt. This usually involves making a huge hole that allows you to try to grab it with a pair of Channel-locks. It doesn't work. Your friend gets pissed off because you can't hold the nut steady. Eventually he gets fed up and goes home.
Do yourself a favor and locate the approximate location of the sheet metal nuts well in advance - use a tape measure or drill some tiny pilot holes in the false floor. When you find them, buy a gallon of PBlaster, drill a hole or two above and to the sides of where the nuts are and start soaking the **** out of them.
Do this for a few days and keep trying to move the bolt. Always bearing in mind that, if the nut breaks free from it's tiny spot welds, you'll be tearing up your cab floor to complete this repair.
A couple of things I have learned about freeing up corroded/seized fasteners:
1. The constant rapping force of an impact driver works way better than simply applying as much force in one direction as you can.
2. An old merchant marine trick to free rusted bolts is to apply torque in the
tightening direction before attempting to back out a stubborn bolt.
I wish you luck.