The rubber isolation ring normally protrudes a bit from both the front and back of the damper. Also, given the age of your truck, some minor cracking in the visible part is to be expected.
That is not what is causing you to throw your belts.
The main reason that belts are thrown - and get eaten up prematurely - is due to misalignment of the pulleys/sheaves that they run on.
There are 2 kinds of misalignment that you should be concerned about:
1. Angular - when the pulleys are not pointing in the same direction
2. Parallel - when they are pointing in the same direction but are offset from each other
They look like this:
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When it is time to set up the pulleys on an engine that has been disassembled (or an engine that has had multiple belt related parts changed), the most critical step is designating a baseline. There must be one "master" pulley that, once set in place, becomes the reference point for all the other pulleys in the system.
I am more familiar with the SBC but the basics apply to an in-line as well.
The best (really the only) pulley to use as the baseline is the waterpump/fan pulley. This is because it's distance from the engine block is "not negotiable". The distance between the front of the block and the waterpump flange face is fixed - by the design of the pump. The pulley is hard up against the pump flange. The fan clutch spacer and fan hub are added on to the forward face of the waterpump pulley.
Once the position of the fan/pump pulley is set, you can start building off of that point. Next is the main driver -the crankshaft pulley. This is normally bolted on to the front face of the harmonic balancer. The right way to line up the crankshaft pulley with the fan/pump pulley is to move the harmonic balancer on the crank snout. Getting proper alignment of the crank to fan pulley can sometimes be a challenge because of the different number of grooves in the two pulleys. If they were both single groove pulleys, you could just check parallel alignment with a straight edge laid across the faces. But in real life, you sometimes have to work from the back or within the grooves themselves.
Anyway, once you have the crankshaft pulley perfectly lined up with the fan/pump pulley you can line up the other auxiliary pulleys.
The alternator pulley will generally line right up - as long as the stock bracket is used.
The trouble comes when its time to line up the other stuff (i.e. power steering pump, A/C compressor, smog pumps, hydraulic pump for a plow, etc.). These components always seem to have a number of spacers/shims that are necessary to get their associated pulleys to line up with the "master group". The "master group" being the fan/crank/alternator.
Just use a straight edge and make sure that the auxiliary pulleys are aligned in both the axial and parallel planes. Recheck after you tighten up a component - sometimes the alignment will change when the mounting bolts are brought up tight.
