When you pull the old tank out take its dimensions (if it's also a 60g). If it's smaller than a 60g then you'll need to take measurements of the coffin. If 60g poly tanks are available in the dimensions you need then you'll have choice of poly or AL. If they don't offer poly in your dimensions then AL is your only choice. If you use AL, search threads here for the modified pascoe method of installation.
The only con I know of when enclosing the transom and adding setback with bracket is the most likely event of moving the COG aft. This will cause performance issues, as AS's naturally sit low in the rear. However, you're removing the deck, thus you'll be able to move the fuel tank forward to compensate for the added aft weight. I'm a fan of the porta bracket, personally.
Console/Tower new is a custom built job. If you're a guide then I assume you live near coastal waters, so you'll have numerous choices on who to hire to do the fabrication.
