First of all, FIND THE CENTERLINE. That's key. Take all the hardware off the body, and find the centerline. Score it on there nice and deep so you have something to work with.
If the neck pocket is too big, you most likely bought an incorrectly-sized neck. Measure the dimensions (L*W*D) of the neck, and draw the footprint out fullsize on your body, after you fill in the neck pocket. Remember, after you route it out, you're going to have to redrill the holes.
If it fits, buy one. And Bygde is right, you don't NEED a template bit, but it will help. And the larger diameter of the cutting edge, the less stress on the router, and cleaner the cut. I got a carbide but for $22.
I don't really see how clamping a piece of wood in the neck pocket is so difficult. My courtesy 1-minute Google search brings up "clamping guitar neck" Just replace the neck with an appropriately-sized block of wood.
But honestly, it sounds like you don't know the first thing about any of this. Read the forums, day and night, if you expect this to get off the ground at all.
You might even be better of just buying a chunk of wood, a template, and a good template bit.