Utility Cycling - oops, can I get narrower wheel hubs?

So I'm building a trailer out of pipe and plumbing parts, and it went together really well. Here's my current problem. I measured some Mountain bike wheels, and got about 3 3/4 inches wide at the bracket holding the hub. So I set up my bars to be that far apart at the center of the pipes. Problem is, the pipes are about 1/2 inch wide, so when I put the wheels in the spokes rub on the pipe :-(.

My solution it to have the wheels run higher than the main cargo area (lower center of gravity that way too), but I'm worried about a lot of lateral slop with a high bracket that's just bolted on.

My question is, do that make wheels that the hubs are less than 3 3/4 inches wide? Cause I could sure use some... What kind of bikes would I be able to salvadge those off of?

I measured one of my EA3(26x1 3/8)rims and the hub is 3"(easily fit in 3 1/2" fork width)wide.You can find these on middleweight Raleighs(or Raleigh produced)or CCM.You could also check the S7(26x1 3/4)rim available on Schwinn middleweights.

Good to know. I didn't know if wheel hub width was a standard thing or if it varied from bike to bike.

There are standards, but there's more than one standard. Sheldon Brown is always a good page to consult on such questions.

Problem is, the pipes are about 1/2 inch wide, so when I put the wheels in the spokes rub on the pipe :-(.
I may not have fully understood your setup, but it sounds like you need hubs where the flanges are closer together?!

By how much do the spokes rub? If it's just a mm or so, you might be able to lace the wheels radially with the spoke heads pointing outwards.
If it only rubs on one side (doesn't sound like it, but still) a disc brake hub has one of the flanges further inwards.