As AK TA mentioned, it might have a crumbling cam gear. I don't remember if the A100 used them or not. Some of the original truck engines around that year I've pulled apart had steel ones. I'd check the chain slop just by rocking the crank pulley back and forth to see how much play there is. I assume this engine is still in the van and is going to stay there? It might be a pain to do, but, you can pull the fuel pump and look inside to check if it has the plastic cam gear or steel one. If steel and the slop isn't bad, I'd leave it alone. If it's the plastic coated gear, I'd pull the front end off the engine and put in a good double roller setup, a new water pump, and reseal everything. If some of the plastic teeth are missing off the cam gear, I'd pull the pan to remove them. They can get clogged in the sump screen and restrict oil flow to the pump. Check the timing cover real good for corrosion too. If you plan on changing the cam, that would also be the time to do it. Just stay mild on the came unless you plan on a full rebuild. Doesn't sound like this is going to be high revving engine, so, I think I'd leave the stock exhaust manifolds on it and just do the duals.
