I replaced the tank in my 1988 21B 3 years ago and SP tanks made the new one for me ($700). I coated it with coal tar epoxy and did the install myself. It was not that bad a job. The toughest part is usually replacing the fill line since Mako foams them into the boat..mine had been previously replaced and I got lucky.
We have always used SP in Toms River. If they tell it is going to take 5 days to build it they will call you on that 5th day and say it is ready. Our shop has done 3 or 4 a year. Every job was different. Sometimes just the deck has to be removed. Some require the console, console and T Top, console ,T Top, and leaning post or chairs. Some decks had to be cut and fiberglassed. Get the job set up with a shop now when they have time. If you can do some of the work great. Most of the labor is grunt work. Take the old tank out, then make your measurments. SP has an estimater on the web site. You can plug your numbers in and get an estimate online.