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i DO NOT in anyway at all know how to read the ID tags on any part of a car, but that big C4 right on the end of the first line, to me, is a dead give away that its a C4.
if you do want a C6 im pretty sure im going to sell mine, its out of the truck listed below, reason im selling it is because im getting a '73 F-150 that comes with a 360 and the matching c6, so im going to use that one and sell mine now, i just got it rebuilt, works perfectly. not sure if im going to sell yet though, depends if i buy that truck or not.
On a C6, one edge of the oil pan is sort of stepped. A C4's pan is basically rectangular. Or for definite ID, the C6 has 17 bolt pans, while the C4 has 11.
Another thing you should look for is that the C4 bellhousing is removable. The C6 bellhousing is part of the case and not removable.
Also noteworthy is that there are different case/bellhousing sizes/bolt patterns for the C6. The one from the 351W WILL NOT fit behind the 390 and vise versa.