Looks like the manifold header threads are stripped - big bummer. Will have a pro install this as it looks detailed along with gas operation, etc...too much to leave in my hands.

Can a HVAC (my air conditioner) contractor do this work ?

Contractor showed me all the rusted out parts (1999 install) and we tried to remove the header - the bolts are rusted out...typical planned obsolesence i guess.

Decided to go with a new Pentair Mastertemp 400 (natural gas non-cupro)...prolly should get cupro HD but selling home next year. $3K installed..not happy.