Chrysler's OEM plugs were either Champions or Prestolites back then. Without looking, I suspect something on the order of a J10Y or J11Y Champion gapped to .035" would be reasonably close to what the original plugs were. Some of those colder Champion plugs might have gone out of the normal sales channels, but I think Mopar Performance might have some in their catalog.
Bosch has a reasonably good reputation for quality items, but when I tried a set of Bosch plugs in my '70 Monaco 383 4bbl in the early 1970s, they did not last nearly as well as the Champions or Motorcrafts I used back then, so that experiment ended. Had a similar experience with SplitFires in my '80 Newport 360 2bbl--they ran no better than the NGK V-Power plugs (in spite of costing nearly 3 times as much!) I had been using or the Motorcrafts that I put in it when I got the car (used). I've even waited on some customers who bought Bosch Platinums and wanted the OEM plugs to go back in their vehicle so I'd run ok again. Personally, I'm not a big fan of Bosch plugs, but have had good luck with Champions, NGK, and Motorcraft as far as lasting a long time and being trouble free. Others in here also seem to like the current Autolites, which is fine.
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