Jeff, what characteristic(s) of the design do you see as being incorrect?
First you've done a fine job so far - these often take me over 20 hrs since there are so many different features in the design
Below you will find a copy of your design with some alterations - Not near perfect just a quick adjustment to illustrate some points
As a note and after looking at a fair share of these I believe and have found evidence of the border going around the whole outside of the design and a matching line between the date and the main section of the stamp. From examples it appears that the date section was separate and replaceable like the two slipped together as on some examples the date section was not fully locked into place and was slightly higher than the rest of the stamping. So would correct that
1- 12 VOLT - was reduced in size
2- (in blue circle) needs IMHO to be bolder
3- MADE IN USA reduced to match the size of VOLT and the whole set of words moved closer to the center
4- FoMoCo enlarged within the present oval
5- Brackets (may have originally been a complete frame or at least a partical frame
6- Agree with Charles the date would be a letter followed by two or three letters
7- And as mentioned earlier needs the C4AF changed to C4ZF or there may have been a C5 or C6 used by mid 66 production
Not sure what the size of the file your working with is. I normally make the design 5-10 times the final sizing to improve the details often (in designs with really fine lines) just submit the graphic file but in simpler designs I just provide camera ready on a piece of paper
Hope this helps