The inside width with a single layer of metal is .625" I had already cut a layer out for an AR15 trigger conversion. Not crazy about how that came out.
Looking at RTG parts they have the trigger groups for the HK G3 for $79. There is an auto sear in that package that you would have to remove and destroy, but anything else that you might need would be there, including several springs. The pack frame wouldn't work, but it might be used for as a pattern for location of the pins.
This idea may even be affordable.