John,
Congrats on getting one of the original Day Sailers! The original boats had keel-stepped masts, so what you may have is a full keel-stepped mast. The length of a keel-stepped mast is about 24'. A deck-stepped mast would be about 22'. The older boats had what amounted to a jack under the mast. It's a large knurled brass disk, perhaps 3" in diameter and about 3/4" thick that threads onto a brass threaded circular post. This post fits into a hole on the keelson inside the cuddy directly below the partner hole. The mast rests on the disk. The shrouds and forestay are attached to the chain plates, and the disk is turned on the threads moving the disk and mast up. This will tension the shrouds.
I may be in the minority on this forum, but I'd suggest not converting to a deck-stepped mast. I don't know how to explain it yet, but converting from a keel-stepped mast to a deck-stepped mast will reduce the performance of the boat. If you have one of the three-digit hulls, then you've got a boat that many in the class association prize. I don't know if I believe it personally, but many say those boats are the fastest. I personally think it's because the people who own those boats have had them a long time and are excellent sailors. In any event, I think it would be a shame to reduce the performance of the boat.
Once you know how to step a keel-stepped mast it's not that hard to do. I've been planning to document the process with some photos. It does take two people, which is a disadvantage for single-handers, unless you can keep the boat stored somewhere with the mast stepped. Sailing the boat with someone else is a lot more fun than sailing it by yourself, anyway.
If you are near a Day Sailer Fleet, find them and go talk to them - Several people in that fleet will be glad to help out. You can find the fleets on Day Sailer Association web page.
http://forum.daysailer.org/fleets2.html
Call the Regional Vice President and have them put you in touch with the nearest fleet.
If you have any problems finding someone, call me. I'm the Day Sailer Class Association Executive Vice President.
(281) 559-2167
Best wishes, and enjoy your new boat!