just to clarify on mine, which was built by harry barlow but i believe still used mec webs and rod (longer rod than yours) it was fully circle too, but upon cutting, i noticed that the mouth needed more than i thought it would due to the angle of the big eye when in normal running, ie with the little eye running vertical. if i would have dry fitted crank with no cutting, it wouldve caught all the way around.
as said, if you own a dremel, this is the easiest option, and will take about a day with alot of crank in and out, cleaning the swarf out each time to avoid contaminating the big end bearing. bit of a pain, but cheap and easy. i would advice having the correct puller and removal tools for the crank due to needing to take it in and out repeatedly. if your drive bearing is fitted, you can use the retaining plate and halite washer to stop swarf getting in, but give the bearing a good flush through anyway just to make sure.
