Tanks, Inc. sells a version of a Buick tank (#B-1c) that appears to have the dimensions of the '50 Olds tank. The filler is at the right place. I can't confirm the rest of the dimensions since they don't show them and you'd need to contact them for those dimensions. Try their web site. I'm hoping to use this tank for my '50 but I'm nowhere close to that stage yet. Don't know anything in the '57 tank.
Bob
Found the Buick tank dimensions that look like they'd work on the '50. 31.5" long, 27.5" wide, 7.5" deep.
Edited by BobOlds (12/29/07 06:03 PM)
Edit Reason: More info.