Just spoke with Safari, the foundry that cast these no longer exists so no record of grade available.
I take it that the foundry was a very small operation.
I recon about 4mm 3/16" at the raised round part.
Only needs to be repaired to work. I had it in mind to run a bead around the perimeter to give me something to work off in case the casting is hollow at the porosity and I blow a big hole there.
Only filler I keep is 5356.
I just checked the chart, 5356 is not recommended for most 300 & 400 series casts , this manifold is likely to be one of them. Might have to get me some 4043.
Got it done, 4043, 3/32" wire & blue lanth, 150amps on the peddle. Cleaned it up, wire brushed, wiped then it turned nasty as expected. Had to flap it back and wipe three times to get on top of the sooty porosity. Don't know how pretty they really wanted it but this is what they're getting. Might scotchbrite.
Thanks. Customer was happy and paid up. Found out a bit more too, a restoration attempt was made on it by sandblasting which removed the thin film covering the porosity.
Bet they won't do that again.