Is there anything special about this piece of wood other than it was given to you from a friend? I mean, are we talking about a $1500 piece of specially marbled English Walnut, or a $75 blank of American Black Walnut?
You are about to invest 100-180 hours of time in this project, (value your time how ever you choose) and it would be a shame to be ever so short when it comes to the wood you need for final dimensions. Dave's measurements of 1 15/16" across the butt plate doesn't leave you a lot of wiggle room if there is any slight miscalculation, mis-measurement, or ever so slight warp in the blank. Dave has built scores of guns, and he REALLY knows what he's doing, so I would certainly trust him to do it right, and, more importantly, to NOT start a project he didn't think he could complete properly with materials on hand.
The other thing to think about, (and particularly with walnut) is grain direction, particularly through the wrist. Figure is nice, but you have to understand that it is grain direction that determines shear strength. Walnut (and particularly Black Walnut) does not have the shear strength that maple (and particularly sugar maple) does. Dave2rc had a very ingenious way to deal with this in one of his builds by inletting a big mortice under the rear foot of the TG, and then epoxying in a cross-grain tenon with some particularly strong material. Another way to deal with it would be to drill a long hole from the back of the breech through the wrist, and glue in a metal rod right down the middle of it.
I'm working on a build now that needs 1 3/4" final width, and started with a blank that was 2 5/8 thick. There was a slight miscalculation in barrel inletting, or perhaps warp in the board, and I needed all but about 1/8" of thickness to get my 3/16" of cast-off and place the butt plate properly