The topic of replacing an aluminum tank has been discussed a number of times on this site. I have a 23' Scepter with a 140 gal. tank and yes I intend to pull it, at least. Probably replace it as well. All of the articles here, that I recall on the topic and the pictures I have seen, show a plywood "shelf" set between the stringers and sitting on the hull. Most of these "shelves" were glassed, or certainly their replacements were when the orginal support shelf rotted out.
As to HOW to install it, an article that disusses the issues involved and explains a "preferred" method can be found at the following website.
http://www.yachtsurvey.com/fueltank.htm
Good luck!