The 3970010 is a casting number for a 69-79 350 cubic inch (and also a 69 302) motor. The single letter C indicates a 57 265 OR a 58 thru 64 283. This should be really easy to sort out......could the "C" possibly have another letter or two more letters?.....but there is no two-letter code beginning with C that indicates a 350. Is the pan off? If so, if it is a 4 bolt main, you have your answer. Is the block drilled along the top edge (just under the head surface) for the plug wire looms? Does the engine have a road draft tube in the valley? Pull the pan and look....the bore will give it away.

mike in tucson