Get the '67 tank. It will fit just fine. Count how many vents your existing tank has, then order one w the same count. Here:
vvap or here:
67-73 Mopar C Body Fuel Gas Tank 2 Vent Pipes CR20B Spectra Premium
OR
Mopar Fuel Parts|Restoration Parts|Restoration Gas Tanks|Jim's Auto Parts
They all run about $300. There's plenty room under your '66 for cheaper, smaller B body tanks too, if you don't mind slightly reduced capacity. I got lucky when I bought Mathilda. Her tank is a proper, full sized 2 vent job without a solitary spot on the interior surfaces! I had to replace the filler neck grommet, which ran around $15, then new 5/16" rubber fuel hose to connect to the steel lines. I made the potentially DEADLY mistake of filling up plum full after that grommet was replaced withoput doing the fuel hose. It busted apart with all the weight and pressure of a full tank, leaving me, wife and baby with a growing puddle of explosive poison under the car. I screamed at the wife to get them away, and crawled under, doing the Dutch Boy at the See Wall act until the Fire Dept and my wife could return in a cab w a new hose. All of which is to say, remember to TAKE CARE with your vents and lines!