Wow, glad I saw this thread. So you need to install a new pisser upstream of the fcv correct? And what kind of 'restricter' does this line need?
You need a T upstream of the FCV, with the T going to a pisser. This is so that your cooling water can always flow through your engine. If the FCV is shut (say at lower rpm's), then your engine is not flowing water and can cook itself. (obviously this depends on your cooling line setup and assumes just the one cooling line / one line off the head).
But, once you have that T and pisser, ALL of your cooling water will want to exit the pisser...it's the path of least resistance. This basically drops the pressure pushing on the FCV to zero, and so it will never open. That's bad. So, you've got to restrict down that pisser line which will increase pressure in your T and FCV. It also increases pressure in your whole cooling system, and can make your b-pipe squirter holes work better too.
So, the trick is to reduce the pisser exit to build pressure...but to not restrict it so far that you are stalling out flow and not allowing your engine to get rid of it. If you have multiple pissers you have to balance this pressure and temperatures for both pissers / cylinders.
My understanding is that the water temp coming out the pisser should be about 120 deg F. Hot enough that you can stand it for a few seconds and that's all.
Usually that means a restrictor of approx 4mm ID at the pisser. You can get white plastic standoffs in the specialty bins at Home Depot. They are about 1/2" long, and fit perfectly into a 3/8" water line. They also have a .17"-ish ID which is just a touch bigger than 4mm. Insert that in the water line right at the pisser, and let the pisser's barb push it into the water line.
