IMO, the neat thing about the caprice tanks (and others) is that the filler exits to the rear.
check out various make/model/year tank dimensions here (scroll down and click on the manufacturer):
http://www.gastanks.com/GAS-TANKS/departments/1/
sorry, they don't seem to have the plastic one mentioned by jayoldschool, but the link can be used to find tanks with the dimensions you need. most guys seem to use the blazer tank but it hangs down kinda low