The pistons will hit TDC twice during operation, once on compression (valves closed) and once on exhaust (valves open). The cyliders are paired, so when 1 is on TDC, so is 6. But they are on different places in the cycle, 180 degrees out from each other. Trn the engine over 1 turn, then #1 should be on TDC compression, valves closed.
On setting the valves, they should be set with both valves closed on each cylinder, preferrably with the engine hot. If setting cold, you do need to leave them a bit loose, about .005" more than the hot setting.
Two questions, do you have a shop manual for your truck? And possibly one that covers the engine? If not, you can look
here for information. This site is free, and you can print the pages you need to take to the truck if necessary.
