I just made the same switch on my '57 235. The recommendation that came with the unit was AC R44SX. I had a time finding them, though. The counter jockey at one of the nationwide chains told me that AC was "phasing out" that plug and gave me a "suitable alternative" (R43S). Well, I couldn't gap the alternative to .074, so I took them back and found a couple of sets of R44SX's at a local parts house. It runs great with them.
