Those were done with two 3.5" pipes into a single 5". Personally, I doubt downpipes will be your concern on the build. I'd be more worried about proper pressure ratios between stages and surge. Especially having it non-gated, you're going to have issues getting each charger into it's perfect operating range. I think it can be done with some trial error and look forward to seeing your results...
Good point, I forgot about it going to 5". I still doubt it will be an issue though.