If it were me in the same position, the first thing I would do is hook up an
air hose, pull the plug wires (don't mix them up, label them in case you
think you'll forget) and clean the dirt around the plugs. Here is where I
would remove one of the spark plugs and make sure it is the correct part for
that truck. If someone had put the wrong plug in that truck and it was just
1/4" longer than the original, the engine would not turn over. Even if just
one of the plugs were longer than the other, there is a possibility the
engine would not complete a stroke. Next, I would take a telesoping magnet
and fish around inside the cylinder bore for any foreign matter. Could be a
screw, nut or bolt stopping the piston from completing it's cycle, that is
small enough to remove through the plug hole.
If you check them all and come up with nothing, then I would start removing
the valve covers and work my way to the heads.
BTW, was the engine you are swapping into the truck known to be running
before it was removed from its original vehicle? Were you able to start it
up and hear it running? What I'm getting at is that if you know the engine
were running beforehand, then you can pretty much check off some of the
obvious items such as seized water pump, broken or jammed timing chain, etc.
Otherwise, for all you know, the engine may have been toast before you even