The sensor has to be pretty cheap. It's worth a try. There has been a lot of 'advice' on this issue since I last checked...I have a much later Alfa, an 86 S3, but except for fuel injection I have the same setup as you, including fresh head (1yrs ago). The stock fan, new, granaded when I hit a dip in the road. New motor mounts were...what we all get when we order motor mounts. E-fan does just fine, using a relay and a sensor screwed into the lower left corner of radiator. Turns on when temp gauge is just over 175 mark. My fan is a puller in back of the radiator, just like yours. I'm running Prestone and some old Sierra I found (didn't buy enough prestone) and just replaced the radiator with a used one in March due to leaking.
I think that you might be OK. I'm hoping that you saved the Evans, as that stuff is expensive but it does the job fine, and you can re-install it later after purging your system. I still haven't installed mine, as I'm saving it for my 74 TR6, and I'm waiting for the small leak in the radiator to become worse before I replace it and purge the block. I've not met anyone in the British car shop that has an issue with Evans.
I don't have a shroud for the reasons that you don't have one. I hate to offer any more confusing advice...but I like the earlier advice given about checking your fuel mixture. You will find the issue, I'm sure! We always do.