That looks like a fun project -- a perfect fit for it's own thread in "Some Assembly Required". There are a lot of very helpful people that pay attention to that thread. To answer your question, there is an engineering saying "if it's stiff enough, it's strong enough." Which basically means that if you affix everything and you load it cyclically and nothing flexes, it's probably strong enough. You want to pay close attention to how thick those stamped sheet frame members are: if you load in the middle of a piece of flat sheet, it will flex and crack. Adding a sheet doubler and/or gussets will help prevent that. My recommendation is to buy the Carroll Smith book, "Engineer To Win". It was written in the '60s, and he speaks very plainly (i.e. not a bunch of math) about good design principles for chassis building. Worth every penny.