25 offset can work as others have surely made them work. It'll take a bit more effort as they still swing the tire a bit more, causing inner fender liner clearance issues. The 275/70/18 is a conservative tire (~33"), so it'll likely work just fine with some cutting of the inner liner and removing the mud flap.
Popular spacer sizes come in .75", 1", and 1.25". These equate to ~19mm, ~25mm, and ~32mm. The math is 60mm offset of stock wheels, minus the spacer size, gives you the effective offset:
For a 1" spacer: 60mm - 25mm = 35mm.
FYI, offset is not just about fitment. Offset is integral to the suspension geometry, especially at the steering axle. And it is correlated to the diameter tire size one wants to run. It's the relationship between offset, scrub radius, and contact patch.
Optimal for stock ~31.2" tires is ~60mm. Optimal for ~32.7" tires ~50mm. Optimal for ~34" tires ~38mm. And so on as you can roughly project out the offset linearly for increasing tire size. There's some room to deviate, but going too far will effect ideal suspension performance.
Too much or too little offset results in the steering pulling, under braking and acceleration. If you've ever driven a high power FWD car, that's what it will feel like. In rock crawling, you'll get more kickback and/or resistance to steering over surface irregularities. The more on deviates from ideal offset, the more pronounced the effects are.
