Used within a VERY specific range of use, it can be incredibly strong, but not for what you're considering. Not to be a wet blanket, but I don't thing conduit will work that well as a rack, either. I would think that the weight of a spare tire would eventually bend it, what with all of the bouncing and etc. that your vehicle goes through. Gas pipe would work better, but then you're back to welding. But, you can buy a 110volt small mig welder pretty cheap these days. I think you could get a non-name-brand for $100 or less. Then, get some scrap pipe and practice for a couple of days. You'll get it figured out, and then you have a new tool and a new skill!
Oh, and JB Weld DOES NOT hold up to race gas. There's a (funny) reason I know this...
"Damn Everything But The Circus..." e.e.c.