Zac_F71's 71 Demon

The champion plugs for those heads are rc12yc.
You could cross reference that number.
They are 3/4 reach 14mm plugs.They used that style plug in Hemis and small blocks.
I run the autolite AR51 in my 496 Hemi. Great plug but expensive.13/16 Hex on mine you would probably want a 5/8 Hex