31/10.50/15's

Hi everyone. Was just wondering if I could rack people's brains (especially those with experiance of this matter!) about a little tyre problem. I have an '88 3.5 EFI RR with a +3" lift, I want to put 31/10.50/15 MT's on, but a friend of mine says this isnt possible without modifying the arches, is this true? If it is, what is the biggest I can go without hacking up the arches? Thanks for any advice, Chris.

**UPDATE** sorry people, the MT 31/10.50/15's will fit ok cos thats what I have on there now!! I was wanting to upgrade to MT 33/10.50/15's, so will this be a problem??!