Most people use either bolts and nuts or pins to secure the mech in the stock.I prefer pins myself because it looks alot better than bolts.The stock is sealed with thinned epoxie on the first 2 coats and followed by sanding and more coats of unthinned epoxie and finished with a uv
resistant varnish.the pin or bolt holes are finished the same as the rest of the gun but spar varnish probaly isn't needed.
Here is an article that has helped out a few people on starting there first speargun.As stated in the article it is just a primer to give you the general idea on how to go about it not a manual or measure here and cut there type of instructions . From what I seen you are building your gun a little different than the norm but don't be discouraged .There are plenty of threads on the subject if you use the search function.You can start your own thread
(if you haven't already) and ask for help or ideas from other members but ultimately You are building it for you to use so you will have to build it to your
needs.
http://www.deeperblue.net/article.php/307/41