Spreader change out

[seawolf]

During my rerig in the slip, I plan on changing the old wooden spreaders on my 1962 Columbia 29. through years of wire halyards rubbing against them they are getting pretty narrow torward the ends. I have bought a used set of wooden spreaders that will need to be modified in size a bit. I plan to pull the spreader one at a time while the mast is up and use it as a template to size the replacement. This is a deck stepped rig with single mid-lowers. Would it be enough to rig a line from up at mast head down to a pulley at the chainplates and back to a winch be enough support to offer safety enough to loosen the upper enough to pull the old spreader off and install the new one from the bosun chair? Ofcoarse safety is the main consern. The cost of pulling the stick and the daily yard bill for having it stored on horses during any rerigging is prohibited at this time, even though that is the preferred option.
Thanks for any sound advise in this matter.
Seawolf