Another thing to think about is that the 9x20 will also be wider than the 7.50 or 8.25's so you may have clearance issues between your duals or to the springs.
As to looking stupid, put some blocks under your truck to simulate the height difference and see what you think. If the 9x20's are 3" taller it will only raise the rear end 1.5", not 3". I personally wouldn't worry about the difference in caster angle. It may theoretically cause a difference...but probably not noticeably.
Tad
1946 Chevy OS 1.5ton, 1955 GMC 3100, along with other various IH,Ford,Nash,MGB,.....
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