Those extra strainers are really going to help quiet down the interior. You're going to love having those. I've been doing the same thing, except for using 3M damping material between the strainers and side panels, and welding the top and bottoms.
Have you given any though to adding more roof bows? Right over top of the driver seats is an especially nasty resonant area. I put more bows in and it had a major impact on noise levels.
Just a thought; did you check the vertical seam at the rear edge of the side panels to see if it's actually sealed? If I held a light up to mine I could see light shine right through on both sides - the factory spot welded it and left it at that - no sealer at all! I welded and body panel adhesive sealed those seams.
-It's been such a LONG TIME... 1981 Dodge Ram B250 Custom Sportsman Maxi Van
Bringin' her back - the construction never ends!
It's what you learn after you know it all, that counts...