Have an opportunity to buy a ARB for a good price, only problem is, it's for gears 3.55 and higher. If I were to regear to 4.10's, is there a way to keep this carrier? Spacer?
So it's for 3.55 and numerically LOWER gears correct. What you can do, is get a ring gear spacer (as you said). It's a machined flat ring that goes inbetween the carrier (your case, arb) and the ring gear to make the gear and pinion mesh correctly. If you get a high quality spacer, I don't think you will have a problem with the set-up. Call around, I'm pretty sure any drivetrain store has them.
I don't see why it wouldn't work. Call around, randy's ring and pinion, drivetrain direct, etc. I'm sure they can tell you for sure if it's a good idea.