Over 12 years ago my 1976 15hp had same problem. We installed a new
impeller, new themostat. Then only a drip out the peehole and still
overheating. Finally found the little o-ring at the top of the water
tube had crumpled and blocked the flow going into the head. This is at
the base of the head where your watertube from the foot inserts.

Greetings

Is there a clever trick to make certain the tubes align into those rubber
grommets?

Thanks

BL