Gas gauge acts like it's working but only goes up to about the 1/8th tank mark. New gas tank in car. It shows the needle rise from the very left to the 1/8th mark when the key is turned to the on position. Any suggestions?

Seems like the gauge is working. You might have bound up the float on the sending unit putting it back in the new tank. Make sure it is able to move freely throughout its travel.

Key on/helper in the drivers seat, you at the sender, pull the L shaped connector off of the sender threaded nipple & with a jumper to get in the female connector, ground it to a KNOWN confirmed ground & have your helper holler if it swings over to the F side & have him holler when it is almost all the way to the F so you can pull the ground. I like to ground it all the way up front to the batt neg post with some jumper cables to CONFIRM I'm doing a legitemate test. With the connector still off of the threaded nipple, ohm the threaded nipple to the circular round sender metal flat base.

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

Basic Mopar gas guage is a 5VDC meter..
If the guage only reads low, then something with the rheostadt on the sending unit arm is out of tweak.
The arm/rheostadt should have a full swing from about 80 Ohms to 10 Ohms..
As posted above, many of the Mopar replacement sending units hare flawed mechnisms...

Just my $0.02..