The Edelmann adapter part number 258302 and 258350 will not work. I have purchased these and they are the opposite to what you will need. Part number 45045 from Classic Performance can be purchased at summitracing.com for $10 and has what you will need to connect a mopar master with 3/8-24 outlets to 9/16-18 and 1/2-20 brake line fittings.
http://www.summitracing.com/parts/clp-45045
A g-body master cylinder will not work without changing out the reservior. The only reservior that will innerchange with the g-body master cylinder, and that increases the volume in the reservior, is from a 4 wheel disc brake 1979 Buick Riviera. The 4 wheel drive Riviera master cylinder is 1.0" bore, which is too large for manual brakes on most applications.
Another 7/8" bore option is from a 1978 Chevrolet Monza. This master is designed to sit level with the ground (unlike a g-body master cylinder) but is 100% cast iron and does not have a pushrod retention cup in the master cylinder piston. But it has the correct GM mounting bolt pattern and has the correct 9/16-18 and 1/2-20 outlets for the stock brake line fittings. Some of the time, g-body manual brake master cylinders listed as 7/8" bore, get shipped as 24mm bores which, in my opinion, is too large for manual brakes on a stock brake g-body.
The Dorman part number M39736 master cylinder says it is 7/8" bore, but the ones I have bought have been 24mm. The Dorman master cylinder that has a 7/8" bore is part number M39451. This master cylinder is listed as a 21mm bore, but it is 7/8" bore. Another master cylinder that I know is 7/8" is a A1Cardone master cylinder part number 131945. This is also listed as a 21mm, but it is 7/8" bore from the ones I have bought.
ALWAYS measure the bore of the master cylinder and rear,manual brake, S10 wheel cylinders to make sure they are both 7/8" in diameter.