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

If you're using Magnum 5.2 heads on it (I would if it's just the lower end) run the aluminum deal you have. If it's a set of LA 318 heads, sell the knock off or trade for the 318 Performer and run that. I wouldn't run either intake you have with the LA 318 port head. It will run. Just not how I'd want it to.