Re: 81' C and C 36 spars
In 1981 C&C was still manufacturing a lot of their own smaller parts.
The hatches and many other castings, goose necks, mast collars, anchor rollers, etc. were custom made by C&C. Some of these items are still avaliable through South Shore Yachts in Ontario.
The mast and boom however would have been made from bulk extrusion and cut to length. I doubt that there was a actual mast manufactuer involved.
Many different models of C&C's use the same dimension of spars.
I'm not too sure what your trying to accomplish by lengthening the boom but you could have something custom rolled to fit and sleeved onto the end of your existing boom, or, maybe a boom off of a larger C&C will do the trick, that way your goose neck may also fit as well.
We have three rules;
No bleeding on the sails.
No slobbering on the charts.