Hi Johnny and Matt,
I have just been setting up mainsail reefing on Naomi. Having purchased the kit from Porters a while back, I have only just got around to doing it. I found that the roller supplied fitted flush to the boom at the rear end. I decided that I did not want this because sometimes I like to join the races and put my best sails on.
Looking at the boom, I saw that it been drilled in a previous life, sure enough the holes were in the right places. I personally prefer the reefing to be on the outside of the boom rather than sheaved through it.
The main halyard still ties off on the mast cleat, but the bottom reef eye hooks onto the boom. Hope this helps.