I've read bedding blocks are sometimes used on 1000 yd guns. I'd like to hear from anyone with experience shooting one, having one installed, and comments per BB's. TIA.

I don't have much to add, other than they work great for those extra long barrels. As long as you have the right stock to fit them in, you will have absolutely no shifting.

But on the subject barrel blocks, I got one for ya all to chew on. I plan on building my own block this time around and was wondering.. I am going to be using a 1.3in diameter barrel, now should the barrel block be cut exactly at 1.3in, or should/need I add another few thousands of an inch, and if so how many?

You should try to cut right at 1.300. I think one of the best ways is to either dowel or key the two halves then clamp together then drill and tap for the clamping bolts. Bolt the halves together with .010 of shim between them and bore to size. There it is.
An alternative method is to bolt together without the shim then mill .010 off the top half after boring. When you clamp the barrel in you don't want to over do it. Regards, Bill.