[svandamme]

i just think you're going at it the wrong way
because you're thinking custom not kit, but you keep asking for kit

first you decide what you want to do with it
that determines layout, dimensions
then you look and figure out what you can put into that frame so it fits and the whole thing makes sense

the pic you posted, that's not what i call a tube frame chassis
that's closer to a beetle then to an atom

it would either require extensive custom work to add something on top of it ,or it would be no better then a beetle chassis

could just as well take a beetle chassis and start welding
and mod it for mid engine layout
many kit cars do just that, with VW gti engines and what not

but to be atom like, it's just completely the wrong setup
the atom is designed to be fully self supporting from the ground up
not as a skateboard with reinforcement

gotta think rigid from the start, not flex and then strenghten if you want Atom