Hey,
I'll try to help a little bit but I hope that more experienced people can maybe help you more.
concerning the barrel. I personally cut just after the front sight. You'll definitelly have a bit of barrel visible inside the flash hider but it doesn't bother me that much. The thing to know is that there is normally a solid piece of metal inside the barrel that is just a bit after the sight too, so cutting 1 inch will cut though that solid piece of metal and your barrel will be completely filled at the end. It's of course possible to redrill that or maybe try to remove the solild piece of metal, but I didn't try that.
here is a photo of my cut barrel and an aluminium tube that I'm using as a spacer for the flash hider; the flash hider will come as far as the tube, so everything in front of that will be visible inside the flash hider.
now, for the hole for the scope mount. the best I can tell you is to place on the gun the mail rectangle of the mount, and compare with reference photos. I attached my front grill first as the rectangle will sit against it and then compared with the photos with the help of the parts of the mauser that are clearly visible.
here are my holes, not really to use as an example, it's better than you use your own scope mount rectangle to position your holes.
Now, real advices that I can give you are that the mauser has to be disassembled to do the holes to avoid going through the mecanisms and, the back screw will get in the way of the hammer mecanism if it's not flush inside. You need to ground it in case it's too long.
Make sure to do your holes and threads really at 90degrees and triple check the space between the holes as you cannot start over once the holes are done.
here are the two Greedo killer I finished lately;
good luck.

it's a very fun build to do.