pic: 6wd prototype chassis

[NickE]

5/8" sounds like overkill for me for dead axles. I would probably recommend 3/8" for that situation.

Just looking at that design, 7 sprockets per side of the drivetrain, with one direct driven wheel, looks way overkill. I'm not quite sure on the function of the extra four, but it appears to be some sort of elaborate chain run in order to use one chain per side. I would recommend using two chains per side with a double sprocket in the middle; one chain run from the front wheel to the center wheel, another from the center wheel to the rear wheel. This makes the chain runs nice and simple, as seen here.

1/4" plate sounds like overkill, but in this situation, it could be necessary to use [relatively] thick sheet metal to maintain rigidity in the chassis. If you have the capability, you might consider adding flanges, as seen in these photos. This would certainly make your chassis rigid enough to use thinner plate.