32vld,
Is you engine on or off the truck?
If it is off the truck do as C-Carl suggests.
If it is on the truck, you can try what K-Karl suggests.
My experience is that an old block gets completely clogged up at the rear. As Carl (C) posted, it takes a lot of work to clean out the rear of the block (side to side, top to bottom). Cleaning out that entire interior area will have a significant positive impact on engine cooling.
Regarding chemicals: they are nice in cleaning a cooling system, but they will not clear/touch a heavily clogged block.
