I replaced my fuel tank on my Cal 29 recently. It wasn't leaking but I was concerned about the corrosion in a gasoline tank. I pulled my Atomic 4 and then took the fuel tank out. The tank came out easily and there was no need to take out any bulkheads. The engine removal was easy as well. After taking of the peripherals I but a 2" thick board under the engine. pried the engine up and slid it out on its oil pan.
This is a good time to rebuild your engine or repower since it's out. Also to repaint and work over the fuel, cooling and exhaust systems. I also replaced the tube on my stuffing box.
I had a local aluminum boat builder make a tank to my specs. The builder was a credible boat builder and properly tested it and put on requisite label plates. The cost was about what I would have paid for a pre-built tank and able I to increase capacity to 30 gal by making the front end lean forward.
I had planned to rebuild but am now repowering with a diesel. Fortunately I had the builder put in a boss for a return line which you need on a diesel. I regret not having him put in a hand hole for cleaning as well which is also desirable for diesel as the algae can grow in the water at the bottom of the tank. Think also about how you're going to hold the tank down and the fittings that you may need welded on or that. You may also want to think if you want a fitting for a fuel gage (I didn't want one). I'd recommend that if you're building a tank have the builder put in an internal baffle to minimize sloshing. This isn't critical but I think it's a good idea. The fuel suction tube should be some inches of the bottom so it doesn't pickup sludge.
Some things with aluminum tanks if you go that way. You need make sure that water doesn't get trapped against the aluminum. I found some design info on line on mounting aluminum tanks and followed that. I built a new platform for the tank to sit on, attached a number fore and aft wood strakes to minimize the surface area in contact with the tank then fiberglassed it to prevent water soaking into the wood which would have the surfaces be wet much o the time. Also it's important that the fittings that you screw into the tank are aluminum or a metal that won't give you dissimilar metal corrosion problems.
I reused my original Volvo wet exhaust muffler. Had I wanted to put in a new muffler like the Centek cylinder shaped ones I'd have a tight fit with my larger tank