Blue Flame,I've had the same problem locating plugs for my '55 235, but have seen 43's,44's and 45's occasionally listed on EBay if you want to look there. However, good advice I've gotten from other sources(web) are to use Autolite 85's for 53 to 61 235 and 86(hotter) for Corvette 235. Also recommended are NGK BR6s(resitor plug)
and B6S(non-resister) for 235's. I don't have info on NGK for Corvette 235, but an NGK supplier should be able to help you to the hotter range in the same plug family. Hope this helps some.
