>Not an expert< but I do have an opinion and a couple of data points. First, the opinion-I agree that the starters were likely attached tot the engines already when the engines were mothballed in '77, so a 1977 date code like the one you have should be in the ballpark.
That part number, however, gives me pause. Here's why-
-While I wasn't paying attention to the numbers on it when I restored mine and I can't make them out in the pictures I have here, I did look at my build sheet and Box 30 lists "Start Motor 759". This is consistent with Wallace racing suggesting 1108759 to be the correct part number. For reference my car is an early '79 MY W72 out of Norwood. One of the ones that shipped without the air dam due to supplier issues.
I couldn't find any references to the part number on your case being originally paired with a Pontiac 400. And of course the rebuilder houses know many of the cases and nose cones are interchangeable, and they swap them around without a second thought.
If you have your build sheet, check box 30 and I think the 3 digits there will tell you whether you have the right case (part number) or not. Like i said, date wise, I'd be comfortable with something in '77 which is when the engines were assembled.