20x9 rims w/ 6" BS, tire size q's

[colsoncj]

I'm looking into rims and tires for the truck, and havent completely worked through the math yet, so I'm curious if anyone has any info. I'm looking at a 20" rim thats 9" wide and has a 6" backspace. Originally I was looking at a 305/55 tire which is around 33"x12". laid out on that rim, the back of the tire about lined up with stock concerning the sidewall, and the rest was stuck outward (1.625" @ the sidewall, 1.80" at the tread). I'm thinking about upping the tire size a bit to open up more tire choices, but I'm worried about fitment. I'm curious if anyone has any rims that are close to the width and BS and what tires they are running?

I've done a little digging that shows 285/??? on stock rims (17") clear fine, while 315/?? on stockers might rub (but where?). I'm curious how these tires fit on the outside (clearance to bumper or fender) so I can try and get an idea what size box im working in.

EDIT: truck is a 04.5 2500 4WD

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04 Dodge 2500 Cummins - DD/Tow Rig. & 02 Z06 Corvette - Weekender.
94 Toyota FZJ80 - Wheelin Rig. & 72 Buick GS - Project.