Yeah Jim bite my head off will ya!
It's not like that, but you clearly didn't read this thread before talking about 7018.
I'm far from a crane expert, but I have built one which successfully holds the 4,000# I designed it to and has been proof tested with closer to 5,000#. His 6x2" wheels are what I use on my runway trollies (4 per side) with urethane tires no less. They run real smooth and are load rated to 2,000# each. My bridge and runway beams are rated for a 5:1 or better safety factor even though the casters aren't. Normal working load is well under 500# and anything over 2,000# is infrequent to the point of less than one occurrence per year, so that's fine.
The comment about gravel is valid, but easily prevented with good house keeping. Urethane does handle some grit without causing a problem too. I used them because they run quieter and metal wheels on a 24' track makes some noise.
In the end, we're all second guessing this crane based on some vague photos and the OP's assessment of it's performance. We're not going to be the ones who have to use the crane or suffer the consequences of it's failure, but I do think the majority of people involved in this thread want to see the OP succeed and have a safe implement in his garage. Jumping in with "shoulda done ____" and then having it not even relevant to the materials in use is hardly beneficial to anyone.
Aluminum MIG in a structural application without a mastery of the process is asking for trouble. Welding aluminum at all in a structural application is asking for trouble. I think the OP knows why we have concerns over some of the joints, and that's part of the learning process. Better to wonder about it now than when a load is swinging at you because something cracked loose.
We got a long way without egos here, and I was pretty impressed by that. Lets get back to helping Rob get his crane going.