I have a set of 18 X 8.5 rims by Foose Lusso and would like to put 275X35-18 on the rear, but looking at many websites they recommend a rim between 9.0-(9.5)-11.0.
Have anyone been able to put the 275 x 35- 18 on an 8.5" rim with leaking or fittiing problems?

Has it been done? Yes
Is it advisable? No. That's not a lot of sidewall to be pinching down that far.
For an 8.5" rim, 245/40 is ideal and 255/40 is max. Ideally, I'd want a 10" rim for 275/35's.