Coolant leak -hydro locked engine

Well, you sure have a good sense of humor... LOL ! Sounds like good stuff. I do recall that the original 351C heads had a poor exhaust port shape at the outlet, and some folks would epoxy up the floors of the exh ports to improve the flow. So, there is some good stuff out there and you are on the intake side and behind a seat so, that ought to be better. (The 351C exh port epoxy DID fall out sometimes....)

Heating cast iron parts to that temp range before welding is common practice. I'd have to wonder if the heads would warp a bit though, and would expect them to be checked for the seats, concentricity, and deck flatness.