Ranger wheel offset?

I picked up someone else's project, a 2000 Ranger . It has a Z'd front clip, tubular A-arms, triangulated rear, 2 1/2" bd. It has the stock tires and wheels on it. I'd like to be able to run it lower than what I can now. Does anyone have an idea of what size and offset I can run without rubbing on a slight turn? The truck is supposed to be set up for a 27" tire. Any help would be appreciated.

I have 20x8.5 with 38mm offset on 245/35 tires. They work but a higher offset would be better like 45mm on a 8.5 rim for 20's

 thanks