Your best bet is to just fill and use an emery board to sand the seam flat. The Monogram kit isn't terrible, but does have some fit issues (as you can see). Overall it builds up to a nice model.
I am surprised to hear someone had issues with the suppressors on the Hasegawa model. With proper alignment I've never had any issues with them (in 6 kits, both A's and D's).
Jon