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CTC was used many times 70 350/360 with a 4 barrel and TH400 in Camaros, and Novas then in 74 400/180 4 barrel CA emissions tubro trans in a chevelle and impala also in 78 as a 305/145 with a 2 barrel TH350 in a monza
CTG is a 70 350/300 4 barrel TH400 used in a vette
so to answer your question, CTC is not a legitimate suffix code on a 70 corvette. if it is a CTG, then it is a legitimate suffix code and is a 350 engine, 300 hp with automatic.