My first post…my first vintage Mopar

I just bought a 1957 Plymouth Belvedere. It has a flat head 6 ...230ci , I believe, with automatic transmission and power steering...the first year PS was available or so I have been told. The car was bought new by a man in Center Point, Iowa from Hendrix Motors. He drove the car until he died at the age of 89. It was passed around to a couple of his daughters, making a few trips to Arizona and back. It ended up in Sandusky, Ohio where it sat in a warehouse for five years, then sold to a guy here in town who had it sitting in his warehouse for a month until I saw it. After a few weeks of seeing it, I pestered him enough that he agreed to sell it to me. Came with new brakes, new tires, new battery, tune up and oil change. So far, Iv'e replaced the muffler and made some heavy duty battery cables using some welding cable. The car runs good up until 45 mph, then goes "putt, putt, putt" (I do too, sometimes!). Vacuum leak or bad vacuum advance??? Another issue is with the brake lights. Since the bulbs were ok, I figured it was the brake switch but have discovered that they will work once the turn signal is used...then they work fine after that...short somewhere??? The last problem is that the fuel gauge does not work. It has the electro magnetic sensor. It is receiving current...maybe a problem with ground??? Anyway, it is a hoot to drive. Hoping to pick up some tips and pointers. I'd add some photos but haven't quite figured out how to upload them for this forum yet.