[kchampagne]

Before spending any more money at the shop, I would have a compression test done. You really shouldn't have to change the oil after 6 months unless you've put some serious miles on the road. I surmise that you have some bad piston rings. This could be the culprit for the dark oil and fouled plugs. If you have bad compression, then you are up for a rebuild. If that is the case, you will now have a big decision to make. You will need to decide whether you are "all in" on the 305 or go another route such as swapping it out with a SBC for example. Getting the 305 rebuilt is going to be quite the challenge: both finding parts and finding someone you can trust to do it. Rebuild parts are extremely hard to find, and if your crank, cam, and/or pistons are shot, the task is even harder. I don't think anyone makes cranks, cams or main crank bearings anymore, so you will be looking to find NOS. Rebuilding the 305 will probably be much more money than swapping a SBC. But, I assume you are here at this 6066 forum because you believe there is something really cool about the GMC big block V6's. That is why we all are here!!! Finding a used 305 is also a recommendation.

Now if your compression is okay, I would replace the entire fuel system. Everything. Until the bad gas and sludge is completely removed from the entire system, you will never solve the problems.

If you are still fouling plugs, you can move on to the carb, timing and distributor.

This process should narrow down your issues. And, I agree with everyone, that 11 hours is way too much to spend before determining if the engine compression is good enough to begin with. It could be that the mechanic spent 7 hours of the 11 reading up on the GMC manual on how to fix things.

I hope this helps!