Which Chevy fenderwell headers suit (early) A-body best?

I'd agree with others that the chevy 2/nova headers would probably be a better choice than the shoebox/55-57 headers.

Yes, Mopar's engines are offset a bit compared to a chevy, but you can compensate for that by where you cut the tubes for the new flange.

Its definitely worth doing additional research to see if you can find any layout information on the chevy headers. Maybe see if you can find any gasser or vintage drag race forums that may have users that have these installed in a car and can take some measurements. You might have to move the master cylinder a bit to allow more space, but a machined plate could create that move without too much hassle.

Another thought, Speedway does list big block fenderwell headers as available for order from Hooker. Since big blocks have the same siamesed center ports as a small block, maybe use these with a new small block flange. Or maybe Hooker could provide some more detailed dimension information. Of course they are around $800, so not exactly cheap to experiment with, and I'm not sure of compatibility with an early A engine bay, but at least they are configured family to family.