There actually is a disc brake setup now for the 3600/3800 trucks.
http://www.ebay.com/itm/350973126802?item=350973126802&viewitem=&sspagename=ADME%3AX%3ARTQ%3AMOTORS%3A1123&vxp=mtr
http://www.67-72chevytrucks.com/vboard/showthread.php?t=586788&page=4I'm pretty sure any master cylinder/brake booster kit will work on the 3600 trucks also.
There was a member that was making dual master cylinder conversion brackets. Can't think of his name off hand. Someone may chime in soon.