The 58810 is a multi-protocol decoder; you don't need a multi-protocol. Save yourself a few bucks and get the 58820 (DCC only) if you want to use an 8 pin plug.
However, what I'd do (and have done) is use the #58823, wired LokSound 5 DCC micro and a KA-1 keep alive from TCS, along with a 13x18mm "cell phone" (a/k/a sugar cube) speaker in its own enclosure inside the tender. To fit all this in, you'll have to take out the original pc board and hard-wire everything instead of using an 8 pin plug with the old board. You don't need the old board, and hard wiring will take about an extra 15 minutes once you know which wires go to what; my recollection from many years ago is that the board is marked, but you might have to do some wire tracing to figure everything out. You may be able to fit everything in even without taking out the old board, but I doubt it. Adding the TCS KA-1 keep alive is well worth it, even with a loco this large that has 24 wheels picking up power (use the TCS KA-1 in all my steam locos, including the Athearn Big Boy I have).
Big Boy sound file is what I used, along with the SP Daylight whistle, but the Challenger file also works - actually pretty much any "heavy" steam file works, since you can set a CV to enable articulated chuffs even on the standard 2-cylinder files. You should pick your file based upon which one has the best rendition of the SP whistle these days. It's been five years since I did this to my AC-12 (using a wired LS Select micro back then), which I sold a couple of years ago.
John C.
