I ran into this problem this last summer. I had purchased a new pair of road shoes (sidi) from steepandcheap...heck of a deal. When it came time to use them for a race, I found that my SPD cleats didn't work out.

I brought them into REI and one of the guys in the cycling department told me about some shims that were often used with another brand of cleat (crank brothers). He went back to their bike maintenance area - found a set of the shims and using those - they put the cleats on for me. It was a tight fit, but it has worked out great. I haven't had any problems with them.

I'd be happy to take a picture of the set-up if you're at all interested. Might be worth trying it before you take a grinder to your stuff. You can always return the crank brothers materials if it doesn't work out.