Brit I think there's an error in your workings. I think what you're trying to achieve is a primary pipe internal diameter equal to the minimum outlet diameter. Given that's been measured at 27.5mm, I'd say that's you primary pipe internal diameter right there. No need to convert to area, as everything's round (ish). If you assume a 1.6mm wall, then you have a pipe outer diameter of 27.5+3.2=30.7mm. The next biggest imperial size will be 1 1/4" (inch and a quarter).
As mentioned, be aware that the manifold stud spacing limits the pipe diameter, and the std 1.7 manifold flare out once clear of the studs. To be honest, for a std 1700 road car the std Alfa exhaust is probably the best bet. It's a large (2"?) system and most seem to have been replaced with smaller 1.5 size units. I'm not a fan of crossover manifolds as they alter the sound to something less boxer like. Personal taste though.