if u havent tore the head apart yet, try black pepper, and a new ceramic sealant to dump in the rad coolant for re-sealing blocks.
if u do blk pep make sure u bypass the heater core so no water run in there, if u mess/plug that up, is not going to be happy too.

I have done many 6 cyl, 5 cyl dsl with these tricks.

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