340 Gets Terrible Gas Mileage

I wouldnt think twice about some old stock plugs.
Right. What yall gotta understand is, it's been years since I was on the line, so I'm not seein some of the stuff yall are since I don't do it on a daily basis any longer. I'm gettin ready to re-plug Vixen's slant 6. What do you think would be good? NGK?