[cadillac512]

Since the heads are on the engine, the way I would proceed is to take a piece of wire and make a 90 degree bend at the end, maybe a quarter inch long...so it looks like an "L". Stick that in a spark plug hole and pull up against the bottom thread, and mark the top or pinch it right at the top with a pair of pliers. Remove it and measure how deep the threads are, and choose a plug with that thread depth, or use multiple spark plug washers to shim the plugs up if necessary.

If you post the measurement on here someone will have suggestions for a plug brand and number that will fit.


Terry


p.s. The carb jetting may be correct as is...but that engine sure would like more carb!