Larry, the Saber PROP EXTENSION I have looks more like a "spool piece" the length you spec and with flanges on each ends, one flange is hard bolted up to the crank flange and the other end is a flange that fits your prop with pressed in lugs to receive your shorter prop bolts. Like shown on the RV4 pic above, but not quite that long. We used PROP EXTENSIONS like that on a lot of Long EZs and never heard of any issues. But every thing I see now has lugs pressed in the prop end flange so you dont have any nuts on back of that fwd flange
This arrangement in place of the 2 1/4 inch PROP SPACER used on a lot of RVs where the Prop crush plate bolts travel thru the spacer and bolts into the lugs that are pressed into your Crank flange. And In this application uses 2 1/4 inch Longer bolts than you would need if bolted directly to the crankshaft flange.
The 2/14 inch Spacers have been used extensively on RV and work quite well.