Either your car was assembled with the wrong tank or someone switched it out at some point. I just spoke with several TB guys and all said they should have the same baffled tank. Here are pictures right from my 1987 GM parts book. Same part number 22523080 for all 4 model years.
Mine is an '86. '86's have different Pump assemblies and different tanks. The parts books are from 1991, so they would have the superceded '87 part number. The tank is "baffled" but is only baffled front to back and not to the sides.
My Tank and pump were 100% original for a February '86 Car. My '86 GN with 25K miles had the same tank and Pump as my '86 T-type with the pump wiring different from the '87 (the same as an '84-85).
Racetronix was selling '87/86 pump kits and had to make a new kit for '86 cars after I bitched and showed them pictures of my 1986 and other people chimed in.
Some people that owned '87 GN's think they know everything there is to know about them. If they haven;t owned an '86, I take what they say with a grain of salt.