What is the resistance of the sender when full? Less than 10 ohms is needed to deflect the gauge to full.
If it is more than 10 ohms, start with checking the ground to the tank and sender, If the ground is good, it could be a bad or incorrect sender. Lots of Repops are just Ford senders in Mopar clothing. Ford needs around 12 to 10 ohms for full scale, Mopar around 8. Not a lot of decent Mopar senders out there. Check with Dixie to see if he has any of your style left. When he had them, they were the best.
Craig