Thanks for the input so far...
Here are the critical dimensions:
Sidewall to inside of tire--3 1/4"
Sidewall to outside of tire--8 1/2"
The picture below doesn't quite line up well enough to show these accurately, but you'll get the idea.
(I guess that makes my tires 5 1/4" wide

)
Sidewall to outside of hub plate where the wheel bolts on--4 3/4"
The fender is 7 1/2" wide, so with the fender centered on the tire, I should have 2 1/8" between the sidewall and the fender as it sits now. With the outside of the fender closer to the outside of the tire, it would reduce that to probably about an inch. I'd probably need some input as to vertical separation between the tire and fender if I placed it there.
Some pics...
I talked to my brother (the one who welded the frame for me) a while back about shortening the axle...he said he could do it, but he was far less enthused about that than welding the frame to start with.

It's a solid axle, with the spindles turned from the axle stock. We'd have to do some fancy cutting and welding, possibly with a square tubing sleeve around the center of the axle. It wouldn't bother me as far as spring placement...I'd just drill new holes for the alignment bolts in the springs. Had to do that already, actually, for the trailer width, as well as the fact that I placed the axle on top of the springs instead of underneath. You can see the original hole in the second picture.
Automotive wheels might work, but it's a four-hole bolt pattern...don't know if I've ever seen an automotive wheel with that. Wheels, tires, AND hubs would probably break the budget right now, although that could be an option down the road.
Also, FWIW, the axle has a little bit of bend in it for toe. You can actually see it in the pic that Steve posted. (kinda scary that you found that, Steve...

) Not sure how critical that is, but shortening the axle would probably change that geometry as well.
Hopefully that helps...
David