I havent had to trim the rec for the recoup to fit in any of my 42's but I did have to cut a hole in the recoup for the fa bolt blocking pin to stick thru. with the clearance hole for that bolt blocking stud the recoup should drop right in. with the front stud out of the recoup the sqaure window should line up with the hole in rec on the left side. The other mod to the rcoup is when viewed from the rear in firing position you have to cut about 1/8 inch of hieght off the right hand upright at the rear so the cocking lug welded on the semi bolt can clear.
I think Pirate or gunslingerdoc on the 42 group was working with the g3 fcg last year. I have had 100% success with the arfcg in my alum grip housing , unmodified ar hammer and hammer spring has ignited every .308 and 8mm I have run across