Jury rig for halyards

[seawolf]

Hello all,
For future cruising plans or voyaging offshore
I was thinking of possible jury rig scenarios and the failure of the external jib or main halyard sheaves was one on my list. If the jury rig comprised of a block attached at the masthead, in case of the main halyard sheave, I would worry about the headboard of the main being pulled away from the mast if at full hoist, and a poor lead at the top in the case of the jib halyard.
Since the halyards ride over two sheaves, should over size blocks be used in a jury rig scenario?
Are there any other considerations in this sort of jury rig?
Thanks in advance.
Fair winds.
Bob