Looking good, I just have a suggestion or three...
It looks like you need some triangulation in the front and rear suspension to prevent bending of your swing arms or failure of your welds when they are put under lateral loads (like when you're turning), bracing this will tighten up your steering by taking the give out of your current configuration. You can follow the same principals as when designing a 3/4 link type suspension. You're not using misalignable links (you're using a rigid swingarm), but the idea's work the same for making your swing arm rigid.
Fig. A are two of my suggestions for triangulation for lateral bracing, the GREEN option would be the most symmetrical and offer more strength from lateral loading in either direction, but the BLUE option is more material efficient, give you sufficient bracing, and would likely stand up to anything you'd be throwing at this cart. Alternately you could take the brace from the BLUE example, mirror it and make an X-type brace which would be stronger than either the BLUE or GREEN suggestions but that would likely be overkill for the application. Triangulation will take the strain off the welds and put it on the material. Without it you'll likely see fatigue/cracking at your welds over time, or an outright failure if you, say, take a corner to hard.
Fig. B illustrates what might happen to your front swing arm under lateral loads(denoted by the red arrow) without any triangulation in place. Without said triangulation you are creating a lever between your lateral beam and longitudinal links which could easily over strain your welds and possibly your swing arm pivot points, and might bend the links outright depending on the quality of your welds and amount of latteral loading you'll be seeing.
My second suggestion would be to provide some triangulation to the rear swing arm. I can't edit a photo but I suggest you start the brace just aft of your pivot and run it to near the bracket holding the middle shock. Then do the same thing on the other side. This will help with chain alignment and stiffen up the whole assembly.
My third suggestion is in regards to your roll cage. I would put another cross brace between the horizontal parallel tubes at the point when they start downward towards the front of the cage. I would also add some triangulation to the front of the cage, there's several places to do it, I'm sure you can figure it out. Also, I can't tell from the pics, but if there should be some triangulation at the rear of the cage as well.
You want to stay away from squares and boxes for anything you anticipate to see any load, this especially means suspension components and roll cages. Triangles prevent a cage from folding over and from the looks of your current cage, folding is exactly what it would do if someone managed to tip it. A box might hold up fine to a uniform load normal to its sides, like dropping a square flat, but drop a square on a corner and there will be nothing stopping it from deforming as you're applying levered load directly to its joints (corners) and all but bypassing the compression/tensile strength of the material. A triangle wont put levered loads on a joint and will rely on the materials compressive/tensile strength to distribute and absorb the shock/load.
A little lateral bracing on that steering column couldn't hurt either. Looks like it might have a bit of side to side play at the wheel as it stands now.
Anyway, hope this helps, and good luck. Looks like fun.
Welcome Visitor! Please take a few seconds and 
) seat 1, and have a full roll cage. It will roll on 14" donuts we got at the scrap yard for 30 bucks and has a leather friggin seat. cool. Buildup Pic's will be posted ASAP.
