The easiest way to find TDC is with a solid stop in the sparkplug hole you are wanting to check. You will have to back the motor up to hit the stop from the back side and mark you balancer, thin hit it from the correct rotation direction and mark the balancer, then split the marks for a true TDC. It's a lot of effort for all six and after a few, you might see a pattern for the rest of them. You should be able to see each piston through the sparkplug hole, that should get you close enough.
Try running the engine at a high rpm and pour some water down the carb, not enough to kill it, but enough to steam clean (shock) the rings and piston tops. A small steady stream is a thats required. Once the truck is really road worthy, go for some high speed test runs to clean it out some more.
Joe
