A problem you'll encounter is this: The earlier SR-25 barrels of your vintage have a larger
outside diameter than the
inside diameter of the barrel nut used with the later KAC railed forends. This means metal will have to be removed from one or the other.
I've swapped out a few. Never again. King Kong could not over-tighten stuff like KAC does. And watch out for Rocksett on everything down to the gas tube roll pin on the Mk 11. You'll be soaking it in the tub for a week to soften that stuff up.
My suggestion would be to forgo the project entirely. Keep the SR-25 "as is" as a collector's item, which it surely will be some day.
If you insist, Marvin Pitts is your (only) man for this:
[email protected]