Dan,
Try this: Set the prop up straight up & down. Measure the distance from both prop tips to the stab LE at the root (blue lines):


On your P-51, those two measurements should be very close to the same if the engine has about 1 degree of downthrust. If the bottom measurement is significantly more than the top measurement (>3/16"), the engine deffinantly has up thrust.

Be sure to turn the prop 180 degrees and re-measure to cancel out any variation in the prop tips.

Check the left/right thrust in much the same way with the prop horizontal and use the TE of the rudder for the reference point.

This is a very rough method of checking thrustlines, but should at least get it in the ballpark.

HTH,
D