gas gauge reading when full but goes down quickly

Maybe give this a try?

http://www.mgexp.com/article/fuel-sender-adjust.html

I really wish I would have found this sooner...just replaced a tank in my 66 fastback and it was a nightmare for a first timer. Probably won't even register >.<

Here's hoping you have one of these magical stop tabs...I guess you could adjust the sending unit to match what your gauge requires.

I think I would still look into testing that replacement and seeing if it can hit the range, replacing it if it can't be adjusted.