Jeff,
first thing I would do if were me would be to check the balancer to make sure it hasn't slipped as Joe said above. If the balancer checks out good, there is something wrong somewhere for sure. If it spark knocks with the vacuum advance not hooked up, you have something wrong beside timing settings.
I said this before, you can't diagnois timing troubles without a dial back light or at least some timing tape on the balancer. Also, Proform makes a real neat balancer cover which is laid off in degrees which is perfect for this. I have one on my truck. It's a real neat item to have. You need to figure out how much total mechanical advance you actually have. Guessing won't cut it.
Here's the one I have.
