340 iron or Airgap-(knock-off) intake on 273 ?

You didn't mention which 273 you had, the low compression or the high compression piston engine. The original 273 heads are closed chambered heads. The 318 heads are open chambered heads that will reduce your static compression into the 8's and your dynamic compression with that cam into the 7's. Teem that with your Air Gap intake (which won't even bolt to your 318 heads) and you have a disaster waiting to happen.