you'll be working a very very hard metal, I've never widen a plate, what I did do was make a slot in a long / tunnel bobbin that is under the spring.
using diamond bit in a dremel. whats called diamond bits are much cheaper than you may think
an other very usefull tool and cheap. at harbour freight for around $3 they have mini fill sets. perfect for redoing screws or finishing off that hole. buy several sets. you will use them