Spark plug problems?

[bud45]

Had to change plugs again in the truck today...the truck is a '69 GMC with a '65 283 out of an El Camino. The original plugs were R43, which promptly fouled after a week. No amount of cleaning worked, so the local parts guy said, "heck no, go with R45s...R43s are too cold!" and so I did. Problem solved for maybe four months, until those fouled, too and BAD! I took it upon myself to get something hotter ( fave guy has transferred, no one knows squat any more at local parts house!) and the only thing available were R46SZ plugs. I regapped them to .035 (they were originally gapped at .050) and they seem to work MUCH better- it now acts like 25+ more horsepower & smoother running.

Since I do mostly city driving (top speed 45-50, with occasional 65 mph jaunts for a couple of miles), plugs seem to foul more often. I understand high speeds, like Interstate speeds & racing call for colder plugs...so slower speeds need a little hotter variety- am I on the right track?

Has anyone else had experiences like this? It's been a L-O-N-G time since I've had to experiment with heat ranges, and it was on Mopars 'way back then, not GMs...thanks! Bud