I don't think therer are too many of us that have done this conversion judging form hambone1958's posts...
http://67-72chevytrucks.com/vboard/s...d.php?t=278438
Disc brake pads are supposed to gently rub against the disc. The vibration of the rotors gives them the clearance that is required. In your case you may have defective disc pads. I have bought some pads before that had so much metallic imbedded in them that it sounded like it was grinding metal on metal. I exchanged them for a different brand and took care of the problem.
It may also be that yours are not retracting fast enough. I'm sure you replaced all of the rubber hoses including the one on top of the rear axle going to the frame. But, if you didn't those old rubber hoses will swell up on the inside causing restricted flow. In other words, fluid will flow under pressure to the brakes, but is restricted or slow to return causing the brakes to "grab" until it slowly returns to the master cylinder...