I have the exact issue going on with my 68 Road Runner. After buying a second sender and having the tank only read 3/4 when full. I compared the repro with my original and saw that the swing arm sits .75" higher on the repro compared to the original. I'm guessing that the float is hitting the top of the tank before getting full swing. When I get back to the garage I will remove it one more time and bend the arm to match the full swing height of the original. I do not have the numbers in front of me but I seem to recall the original was 7" from the sock sitting level to the top of the swing float and the repro was 7.75". It will be a few weeks yet before I can verify the fix.

Good Luck

Jim
68 Hemi Road Runner