54-tooth sprocket will need a strong engine to turn it, that's a little bit small, but like jamyers said, close to the ground is not good. That's harder on clutches too, but too big and the ground clearance shrinks to nothing.
Wheels, sprockets, and brake discs are always best placed close to a supporting bearing. You've got some locking collars for them there, use them all. Personally I only use DOUBLE-split collars that can be placed/removed in seconds. Main thing is that mine go on BOTH sides of ALL of the various parts because I can afford the average ebay cost of $2.65 each better than affording the cost of warped axles.
Since that angle iron across the rear isn't heavy enough to do the job (more square tube) and the cross brace isn't welded yet, you're not committed yet to that setup. Start over with heavy square tube, and leave more space between tubes not only for the sprocket, but also for the chain possibly running at a good angle away from it.
Consider that brake drum or disc placement as needing either just a brake rod, or a cable, or a hydraulic hose, or combo of those. Simple rod on the left as jamyers said. Cable all the way from pedal to brake. Rod to a master cylinder, then hose to brake. Or any rod and cable combo that works, from a left side only setup, to crossing over the frame left to right.
For that frame, a "rear clip" like you've got there is much harder to secure strongly to a mainframe than it is to start with tubes that run all the way from front to rear.
