Not having done this project myself, but seeing it's desirability, a couple of thoughts. Regardless of the type of block used, a cheek block or a tail block, I would think that through bolting would be essential. Reinforcement via backing internal to the mast would, in my opinion, be essential as well. I would probably be looking at a caged block as well since you're probably considering the sheave being outboard of the mast. That would keep the halyard fairled to the block even when slack. Proper sizing of block to line will be essential to prevent fouling as well.
A padeye could be attached for use with a tail block and would allow a wider use of the halyard at different angles of strain. Padeyes are readily available, but I would not advise their use without proper backing of the mast interior. Screwing is contra-indicated. It will eventually result in the block descending to deck level, at speed, with attendant damage to deck or yourself.(g)
I would think that a custom fabricated plug of aluminum or Delran (sp) would be the best way to provide internal mast head support. Either could be thru-drilled and padeyes or sheaves mounted externally. One advantage to using cheek blocks, perhaps negating the advantage of a tail block on a padeye, would be that it would be just as easy to mount a double sheaved block to either side as it would single sheaves.
For more on cargo derricks I'd refer you to the Merchant Marine Officers Handbook.(g)
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