We fitted jackstays to our B34 which is regularly sailed in the Irish Sea. I would recommend fitting stainless steel U bolts aft of the forward mooring cleats so that the nuts are accessible inside of the anchor locker. The U bolt comes with a s/s plate that fits above the deck and usually has a hard fibre backing plate to spread the load above the deck. The U bolts are long enough to take a 15/19 mm plywood backing pad under the deck, and you need good sized s/s washers on the underside of the backing pad.
The same system is used just ahead of the aft mooring cleats. This position is accessible on our B34 via the cockpit lockers.
The Jackstays are made from 25mm webbing from Jimmy Green, correct strength for the job. Order them short of the total length required by 100mm. Fix the forward end with a large shackle, capable holding the 25mm web on the shackle pin. Use fine high strength line to lash and tighten the aft end. Chandler will advise type and you can get enough turns to make it strong enough. Lead the Jackstays outside of the shrouds so you can go forward without having to unclip.
I would not recommend the use of spinnaker halyard as it is likely to get behind the spreaders and cause problems.
We use a double safety line and if we need to work hands free we clip the second line to any point high up, such as the spinnaker pole ring on the front of the mast, to ensure we will not go overboard.
The set up I have described is the one used on most racing boats and works well.
If you leave the jackstays on all season, tie them up at the shrouds when leaving the boat. This stoppes them gathering dirt on the jackstay and the deck.