Bad head for my 512 stroker

Short block is done. Heads are done, well I thought so. Cleaned, maged, ported, gasket matched, polished, hard exhaust seats, milled to cleanup, paint, & freeze plugs. While lapping valves I found small dot in #2 intake seat so back to machinist (I do most my work). Each time we touched it, it got bigger - POROSITY!
In all the B & RB stroker builds I never saw a defect in the seat area. These are 4006452 castings (76'-78') from a motorhome with the additional 1/8" water holes around spark plugs to match the block.

Yeh yeh, I know buy a set of aluminum & forget it. Not me, i'm hard headed(sic).

Machinist will try cutting for replacement seat but where there is poroisty it usually runs through to water.
Anyone ever run into this before?
Anyone in socal have a spare head to match my good one? If not I will start over with a set of small chamber 3462346 castings I was saving for another 451 B stroker.