I would think you need a smaller than stock size pulley for the W.P.
As a guess, stock size damper is 6.750" you are going to an 8" so, you would need to go down to a size of approx of 1.25" smaller on your W.P. pulley,note: this will speed up your W.P & all your accessories will spin faster also because of the 8" damper.
It won't be 8." exactly because you are going to machine grooves into the damper,so , say, 7.75" - 7.50" or so.
If you do not want to go to the trouble of changing too much, I would think a S.B.C., 6.750" damper will still work, & better than the stock L6 damper, & this way, you would not have to change anything else.
I personnally have not used one, but I would use a SBC one over a stock L6 one.
I never ever had any stock L6 dampers worth a BEEP, going over about 5000 RPM.
If you have an 8" damper already, as a close guess, I think you would need a W.P pulley about 5" - 5.5" OD to clear the 8" damper
MBHD