Have you recorded all the data possible of the good engine on a cold start up ? Should point out some problems if possible / recordable.
Record on a smartphone all the computer running cold and hot data. Compare that to the other engine.
I just noticed you REALLY have at least 2 bad conditions...
I still do not see a checking of flatness of cylinder head surface AND the cylinder block surface ?
If lower Vortec bolt pressures are correct ? Why were so many bolts loose ? Did someone do the ....Double Head gasket fix..... A real looser of a DIY.
Pull it all apart and have a Machinist do it...
I do a COMPLETE coating / sealing of every seam / drain hole / screw head & hole / metal hole with bolt or screw / wires coming out of light. It seals every time.
There should not be a lot of smoke from the slight extra amount of oil in the tank. If there is. You are not measuring the amount of oil correctly. Or you are not mixing the oil and gasoline correctly / well enough.
I know of 4 things to attack metal parts in water.
Salt Water
Polluted industrial / corrosive places
Excessively strong Chlorine sewage treatment plant discharges/
Strong stray electrical currents. Dock power or underwater power cables leaking electrical current into the water.
Success !!
Cleaned out gas tanks and 5 gallon of new 0 % ethanol marine Hi Test. new Primer Bulb. Back to instant starts ....With 2 new gapped correctly plugs. :)(y)(y)
Sounds like the water is taking way to long to warm up. Thermostat is wrong one ? installed wrong ? Choke is wrong ? Missing a gasket under the thermostat ? Temperature gauge is in correct ? A little bit of some problems ?
The key to forever life trailer lights. Is the separate Ground Wire And completely sealed light housings. My oldest trailer is a 1984. I back in with no cooling off of bearings or lights. Day or night lights on.