Nice re-use/re-purposing! BTW, based on what I've read they're definitely necessary and as you mentioned, going without a heatsink can significantly reduce an SSR's lifespan.
Just a tip if you ever plan on more SSR's, I use three SSR's on my HERMS system and had a hard time finding a heatsink that was the right size and didn't cost an arm and a leg. I went with one from Extruded Aluminum Heatsinks from HeatSinkUSA
Their prices seem very reasonable and they have quite a few length/width options (that's what she said). The only con I could find was they aren't anodized but so far that hasn't been a problem.