Boom Vang Part

[TomP]

I would not recommend you follow this.

I understand what you are trying to do. But if you look closely at thr slot, you will find that it is no very thick. Add the point pressures on such thin walls and you could get yourself into some trouble. With your 15 foot boom, and the huge main the '36 has, you could quite easily pull the fitting thru the slot.
I would recommend attaching a hound to the mast at deck level. This way, the hound will be mechanically fastened to the mast with no less the 3 fasteners on each side. One must try to spread the load whenever possible.
If you have an original C&C Spars mast, they were built with an extra 1/8 aluminum bar on either side of the mast wall that runs the entire length of the mast. With the proper size fasteners rated for shear strength you would have a almost bullet proof installation

The C&C '81s came with a deck crown. Thru bolted. Yours does not have this?