Dusty - 778 is not all of the number. Try cleaning the tag with a pencil eraser. Then hold a strong light to shine on the tag about 2 degrees above horizontal from the edge. It throws shadows in the stamped number.

If 9778, then it would be from a GMC 270, and probably is going to be difficult to tune well on a 216.

But not to worry. There are lots of dudes that frequent this forum that would swap you a GMC 228 carb for the 270 carb.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop