MIMF

I don't know how you all do it, but I was thinking about 1 each 1/8" locator pin and 1each 6-32 threaded insert and machine cap screw to hold the pickguard to the neck and a standard Gibson style bracket down near the shatten volume thumb wheel. Im very concerned about a split forming at a screw hole in the neck down the road. What the best way ???
Also how far below the fingerboard should the pick guard be?

There are so many arch top builders out here with so much more experience than I've got. I almost hate to be the first to chime in. But I will. I've got a 1937 Gibson archtop mandolin with nothing more than a register pin to hold the finger rest at the neck end. On one of my arch tops, I used two register pins. On the next, I glued a small piece of wood under by finger rest at the neck end, positioned to be fairly snug to the neck extension. I drilled one hole to accept the shank of a small brass wood screw. I drilled a smaller diameter hole in the side of the neck extension to receive that screw. I glued a thin felt pad (dark brown) to the side of my wood piece under the finger rest to cushion it as the screw snugged it up to the extension. This was to obviate the chance of a buzz there. It works fine. I don't think you need a fancy insert. I think that's sort of overkill. (maybe my felt cushion is overkill, too). But you can do it that way if you wish.

I do much the same as Patrick but use two 1 1/4" drywall screws as per Benedetto. Make sure you pre-drill the holes. The black screws match the ebony too.

Over the years I have used several methods but now I use the threaded metal insert in the fingerboard support plus a pin at least an inch farther back (toward the bridge) to keep the guard from rotating on the machine screw. This is usually sufficient to support the guard, but some may prefer a bracket from the waist area to the guard in addition to the screw and pin.

If you mount a suspended pickup to the fingerrest some adjustment is handy for getting proper placement of your pickup. One way to get some height adjustment is to drill the mounting block with two holes. One round hole and one U shaped hole or slot. Use two threaded inserts in the fingerboard extender spaced to exactly match the holes in the mounting block. With some cork sheet glued to the inside face of the mounting block 2 screws hold it in place. The angle of the pickup can be tweaked by sanding some of the cork away to change the angle of the pickup. Once the fingerrest is where you like it then add a small block of the proper size to support the back of the fingerrest. Put a little felt or cork on the lower face of the block to protect the guitar top. Threaded inserts are the way to go. They don't wear out. But be sure to drill a large enough hole for them. You can easily split the thin wood of the extender. There are lots of ways to mount a fingerrest and I have tried several. The threaded inserts are more effort but they look clean and last forever.

good tips all around.I didnt think about the buzz factor and the cork /felt backing to avoid that.I think Ill eliminate the waist bracket as well and mount a block with cork/felt to support the rear of the fingerboard. I guess Ill try to keep the top of the fingerboard 3/16" below the fret board height any play around with pickup height using foam tape first then mount permente with inserts and tapered cork . If this guitar was for my self I might not go to the trouble of inserts,but its for some one else and I dont want him calling me later to say anything, except that he loves it ya know??

Gentlemen, it would really help if you make a side view drawing of your instrument before you make it. Put the guard and pick up(s) in the drawing along with everything else (strings, frets, etc.) You can plan your over stand to accommodate the thickness of the pick up and some padding, then you can let it sit on the surface of the top, or give the over stand enough height that you can float the pick up just about anywhere you want. A lot here depends on how much clearance between the strings and the soundboard you want.