Newbie with my first Cuda at 62 yrs old

Both. You will find this is one of the most active sites on the web. It also has an amazing search function. Win, win. Where would you like to start?
Well, Since ya asked.....Got the car last weekend. Fuel gauge not working so I went with the most common route to fix it. swapped out the sending unit, and now it shows Something, but before I installed it, I hooked up the new sender outside the tank & had the full range of readings on the gauge in the car, so I'm thinking, easy Peasy, slap the new one in & problem solved. Well, I took out 6 Ga ( should be exactly 1/3)from the tank to the drop, & after everything was back in, put the 6 ga back in. But it's got me scratching my head... reads almost right at 1/8 tank. & ya, double checked wire & ground strap. Any ideas? Like I said, had it hooked to the same wires before I put it into the tank, & had Full range of reading, and now this.