[chuckmc8]

I found a retailer on the web that had this info posted. I just Googled "BOSS 338 fitment guide" and chose one. Although these are 18" I think we determined that the hub area is the same on the 18 and the 20". I just cut and pasted the info below. Notice that the vehicle that I chose was a 71 Blazer, It didnt give a 5 lug 71 truck as an option. These have "gauranteed fitment" I guess that no one has tried it on a truck rotor tho. It does have the other models listed that these are suppose to fit in a 5x5 lug pattern. Any of these GM rotors interchange with our trucks?
OEM tires: 225/75-15 (717x225mm). Bolt pattern: 5x127 (5x5). Offset: -10 to 10 mm. Tires up to 757x265mm. Rims up to 23x9.5".
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BOSS 338 B
18x9.5, Bolt Pattern (PCD) 5x127 (5x5), Offset (ET) -4mm
Gray/Machined Lip, $169.00 each.
5x127 (5x5) bolt pattern, offset -4 is considerable for:
( 18x9.5 size can be unsuitable for certain models/years/trims )
Buick: Roadmaster
Cadillac: DeVille
Chevrolet: 1500 2WD truck Astro Blazer 2WD C10 C1500 454SS truck Impala Laredo 2WD Suburban 1500 2WD Tahoe 2WD Van 5 lug
GMC: 1500 2WD truck Jimmy Full Size 2WD Safari Savana 1500 Suburban 1500 2WD
Lincoln: Continental Mark III

I also looked for Astro van rotors for a pic-notice the step in the hub that is similar to the machined version that was modified for the 'burb