I would not use the S10 master cylinder. Here is a source for the long intermediate pushrod if you can not find one elsewhere.
http://www.ebay.com/itm/181958676080...%3AMEBIDX%3AIT
You can also use a master cylinder for a '70 Buick Estate wagon, which has power drum brakes but the shallow bore in the back of the master cylinder piston. It is A1 Cardone #10-1363