So buy that Matter Cup cage that's been floating around for months, re-jig the dashbar and column support, strip the inside and you're 95% there.
I'd completely scrap the s/c idea, get what you can for it and build the car ready to take a known plug & play conversion further down the line. Megane turbo or Bic's S/C kit once proven. I think your route is going to be 3 times the price and there's a good chance you'll never get it running right. No matter how many times you say "it'll be worth it" and "I'll never sell it", you'll lose interest and want rid, I've seen it a million times. With stuff so bespoke, you can only afford to be the pioneer if you can either do it yourself or you've money to burn.
Make the car faster the Chapman way, by adding lightness. Enjoy for a year and then look at proved conversions once they're more popular and the price has come down.