which front brake calipers

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I installed a ssbc 6 lug disc brake conversion kit # a126-71 on my 67 swb,I have replaced almost every component in the system twice,I have bleed the brakes a hundred times.I finally put a 72 master cylinder which I bleed twice, and a dual diaphragm 8 inch booster from summits on it.It will stop ok but it will not lock up the brakes.every part in the rear brakes is new,It has an adjustable proportioning valve,So I am looking for a set of calipers.The instructions says it takes D-368 brake pads.The center to center bolts on the calipers,look to be 5 3/4 of an inch I just crawled under the truck and did not remove the wheels.any info would be appreciated.this engine has 22 inches of vacuum.Thanks Jeff