Get that little miss taken care of first. Find out what's causing it (plug change is smart, but make sure that's the case).
The HEI swap seems to be pretty popular around here if the problem ties back to the ignition module. And, as I said before - if it has lean burn, get rid of it.
Seeing as your junkyard seems to get old stuff frequently, you might want to keep a look out for 920 heads (closed chamber from some 273s or intro-year 1967 318s) as a quick and cheap way to bump up compression on a stock LA 318. No pushrod or rocker antics - just drop them in and go.
Only problem: You probably won't find a pair of good 920's just hanging around, and if you do, their age may be an issue. I wouldn't necessarily bet that a home valve lapping job would get you out of the woods on 40+ year-old cylinder heads. But stranger things have happened.
-Kurt
