Coolant wars! This is shaping up to be like the posts on Oil types, or if DOT5 is any good...
Your thermostat has the most direct influence on your coolant temps. Evans will run a tad hotter, but nothing like what you are seeing. Given the proclivity for aftermarket spare parts to be crap, you should put your old thermostat back in. They rarely fail, from what I've read on these forums. And given the expense of Evans, the thermostat change out makes the most sense per dollar.
If things work out, another owner in 10 or 15 years will pull the head and have NO issues with corrosion on the studs, and it will come right off! Ditto for water jackets.
Another thought- I'm not sure if Alfa sold thermostats for different climates like most other manufacturers, but you could have gotten a cold climate thermostat, or maybe they don't want to make different climates, so they sell 'one for all'.
I'd put in your old thermostat.
86 spider Veloce