As promised, after searching under the rocks found that Fastenal could get you the original Type AA six slotted head bolts with 300M steel for you GMC flywheel, made either by Aetna Screw or Chandler. Not cheap, 16 bucks each.
Since bolts are 1/2x20 not much options. ARP's cheaper goes to $30 up to $100 (top fuel) for the kit of six in that size. You Chevy guys are lucky with 7/16 bolts. I'll go with the ARP's, but in case your restoring to exact specs or are a little bit anal on original parts, hope this is useful. I contacted Fastenal in Houston.