It can vary, a 2.0 litre is a 4cyl. and a 3.1 litre is a 6 cyl.. If you cant find the specs on the labels on the door or in the engine compartment, you can check the window sticker that came on it...
There are two heavy steel straps that support the tank. I *think* (it's been a few years since I did one) that you may have to drop the exhaust a bit to let the tank clear. The straps have one big...
I think a 1992 through 2005 Bonnevilles hold about 19 to 20 gallons of fuel. To double check this answer check the owners manual and see if it saids anything on fuel compacity