[68gmsee]

Not sure what you mean by "the truck does not run properly until I have advanced the distributor from the stock 0 degrees mark on the timing tab to a point where the distributor mark is almost pointing straight up."

You may just have to adjust the distributor timing until your truck runs good. These trucks have been changed from stock in so many ways these days, that you can't go by the book anymore. Move the distributor until your truck runs to your satisfaction for now.

I've heard that some guys adjust it to 12 degrees tdc and some even higher just to get their trucks to run good. Play around with it and see what you can do.

Also, re: valve timing. If your timing chain is worn and/or your cam worn flat from many miles of use, it definitely will affect the performance of the engine.

It would require you to remove the timing cover to check the condition of the chain.