I do have the Selden mast. I'm not sure what the considerations would be for the Proctor mast. Probably need to understand the internal configuration and wall thickness.
It might be easier to do with the mast stepped if you have 2 people. You would definitely have gravity on your side and the line would fall where it naturally should if the boat wasn't rocking. You would need someone at the bottom with some kind of wire that could catch the line and pull it through.
I uploaded a photo to the Photos section under Bella V2620. Its probably on the last page.
I cut 4 slots in the mast about 10 feet above deck on either side. I spaced them vertically by about 1 foot, and I separated them apart by a few inches. The photo shows only one side, but the opposite side of the mast looks the same. The only exception is that the slots on the other side are lower than those on the side showing. My thought was to have all of the slots at different heights thus reducing the potential loss of strength by having all of the slots at the same point. I'm not sure if its sound thinking, but we've been out in 45 kts and she is still standing.
The slots are about 1 3/4 inches x 5/8 inch. Make them as small as possible to accommodate the exit plate. I cut the slots with my Dremel, then cleaned up the edges. Once the slots are cut, drill the four holes for the rivets and mount the plates with 4200 Fast Cure. Concerned about the bond, sealing and corrosion, I contacted West Marine about mounting the stainless steel plates to an aluminum mast. They suggested using a generous amount of 4200. Since there is no stress on the plates (the line just slides through them), There is no need for heavy duty fasteners.
Before I installed the plates I fished the line through with fish tape (the mast was unstepped). It took a little while, but nothing major. I did it myself the first time because I was mounting an anchor light and had a small hole drilled in the top for the wires at the time. The hole allowed me to see what I was doing. I rotated the mast into various positions to guide the line to the correct side of the various internal structures. Once the line was at the top of the mast, I fished it over the correct sheave and out the top.
Once the lines were fished through I wrapped them in plastic wrap (like Suran wrap) and slid the plates over them and into place. I then applied the 4200 and riveted them into place. The plastic wrap keep the lines from getting 4200 on them. Once the plates are in place it is difficult to see inside the mast and fish the line through. So, it is important to fish the lines through first.
If I were doing this on a stepped mast, I would drop the lines down from the sheave to the plates. In theory, if the boat were not swaying, the lines should drop where you need them. Get a wire hangar to grab the lines and pull them through the slots.
Materials:
Schaefer Exit Plate SC3466
M 4200 Fast Cure
Rivets
Drill
Dremel
I hope that helps. We've enjoyed having the extra lines handy. Although they are not used often, I like the idea of having backups.
Jack
Bella - V2620