Are you certain it has a 9 inch ford and not a chrysler 8-3/4? Run a compression test, and tell us what those numbers are as the type of spark plugs and their numbers. I've seen the temperature range set 2 steps higher than stock (Champion JYC14 instead of JYC12) on a 318 kill the performance even with everything else set where it should be.
If you have "J" plugs in a 318 that could castrate it a bit as they are the short thread ones, the 318s (except I believe the old poly ones) take an "N" plug if using champions.
And I e used RN12s and RN 14s back to back in the same engine (sometimes based on what was in stock when I needed a set of plugs) with no appreciate difference in power) many times over the years.
And even though when you subtract 12 from 14 the answer is 2, in Champion plug numbers that is only 1 step not 2, I don't think they made an "N" or "RN" 13 heat range though I've seen other series of champions in a 13 heat range.
If going from a 12 to a 14 heat range (or vise versa) makes that big of difference in how your engine runs you got a bad batch of whichever ones made it run bad. .or else the ones that made it run bad were just plum wore out.