Installing a new mast without measurements

Hi there, new to sailing and found myself an old Sailstar Explorer 17 (hope that it's okay that I ask for help here, since this is a Daysailer knock-off.)
This boat came to me without it's original mast (which had snapped in two,) but with a new mast from some type of Daysailer that the previous owner had bought with the intentions of fitting it to the boat. He was not sure how long it should be cut, so that part we (he's my neighbor) still have to figure out.
We still have the bottom portion of the snapped mast (along with the riveted-on tracks for the book and the jib) and assuming all else is equal on the new mast (which is 24' 7" long or so,) it seems to be the right size for the Sailstar. The previous mast was deck-stepped, and the new mast appears to be meant to be keel-stepped. When I measured from the slot where the sail is fed into the masts, the difference between their size is pretty much exactly the distance from the floor under the cuddy to the top of the cuddy.
Firstly, neither of us have any experience with this. From what I gather, we not only need to attach the deck-stepped portion of the mast to the hinge plate on the deck, but we also need a section of mast to go under it as a "mast stub" so that it connects to a fixture on the floor which is presumably on top of a compression post under the deck. The hinge plate type thing we have that allows the top part of the mast to rotate does not look like the kind that attaches to both mast and mast stub, but maybe I just don't know what I'm looking at. I can get pictures soon.
But before we go and cut a hard-to-find mast, we want to be sure. Our thought is to lay out the new mast along with all the sails and boom and pretend to pet it all together to make sure it fits. Would this provide a close enough "simulation" to make sure we're cutting in the right place? I've never rigged a mast before so I'm not sure what to look for as out-of-place.
Thanks,
Marcus
P.s. the guy whose boat I bought is on here as "waterbug" and has asked some questions before.
This boat came to me without it's original mast (which had snapped in two,) but with a new mast from some type of Daysailer that the previous owner had bought with the intentions of fitting it to the boat. He was not sure how long it should be cut, so that part we (he's my neighbor) still have to figure out.
We still have the bottom portion of the snapped mast (along with the riveted-on tracks for the book and the jib) and assuming all else is equal on the new mast (which is 24' 7" long or so,) it seems to be the right size for the Sailstar. The previous mast was deck-stepped, and the new mast appears to be meant to be keel-stepped. When I measured from the slot where the sail is fed into the masts, the difference between their size is pretty much exactly the distance from the floor under the cuddy to the top of the cuddy.
Firstly, neither of us have any experience with this. From what I gather, we not only need to attach the deck-stepped portion of the mast to the hinge plate on the deck, but we also need a section of mast to go under it as a "mast stub" so that it connects to a fixture on the floor which is presumably on top of a compression post under the deck. The hinge plate type thing we have that allows the top part of the mast to rotate does not look like the kind that attaches to both mast and mast stub, but maybe I just don't know what I'm looking at. I can get pictures soon.
But before we go and cut a hard-to-find mast, we want to be sure. Our thought is to lay out the new mast along with all the sails and boom and pretend to pet it all together to make sure it fits. Would this provide a close enough "simulation" to make sure we're cutting in the right place? I've never rigged a mast before so I'm not sure what to look for as out-of-place.
Thanks,
Marcus
P.s. the guy whose boat I bought is on here as "waterbug" and has asked some questions before.