if you think its the head
you can pull the spark plugs out of the jugs and inspect the plugs for water/coolant drops, this will tel you if the head or head gasket and what jug is effected
could be erroded water pump impeller or broken pump shaft, this you look for coolant moveing past(bubbles) the fill hole, remove cap...top off , start engine , warm up and observse thru fill hole for flow
no flow? either thermosat, or bad pump
the manifold stuff i dont know, i as outboard man
also be careful, you dont want to scald your face off, when these things are hot, hoses can burst melting your skin off with steam, or the thermosat is stuck it can unstick anytime your looking into the fill hole when the thermosat decided to open , your a cyclops
you should'nt play with it when it;s actually overheating, or within 2 hours till the core of the engine is cool enough, i am serious. you can get hurt real bad, if you dont know what your doing, stay away, not to make fun, but it sounds like your not experenced in cooling system troubleshooting, get a mechanic to do it
pressure test system, when under pressure look for actual physical leaks, i did not read in your posts if your finding coolant in the bilge or if the morot is burning it...where is the coolant going?
this will help diagnisis
1 look in bildge for lost coolant
2 i say pull plugs, and inspect for green droplets, water droplets, or if one or 2 plug is shiney clean, not matching the rest let us know the results
3 following the steps above, warm it up and look for flow, like others posted
4 pressure test
let us know above 4 and then you find the problem easy.