Bigger boost is better with discs, But if you are concerned about clearance go with a dual diaphram 7".
An 8" single has about 50 sq inches of diaphram area, a 7" has about 38 sq inches and a dual 7" has about 70. sq in.
Each inch of vacuum is good for about 1/2 psi, so you will get about 600 pounds of boost at the mc piston from a 7" dual ...450 with a 8" and 340 with a 7" all at 18" of vacuum.
An 8" dual will give you about 850 #.
The 8" single should be alright if you don't mind pushing the brake a little harder. If you are using the TCI pedal assembly for that truck, then it is a high ratio arm anyway, and your brakes will be easy enough...(about 6 or 7 to 1 IIRC.)
I've installed the boosters with only a couple of inches clearance with no problems.
Edit..I didn't see the post above where you said you were considering dual diaphrams.
If you are using GM calipers use the 1 1/8" master.
Hope this helps,
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Last edited by powerrodsmike; 02-16-2008 at 11:47 PM.