BTW, are you going to do anything about the fuselage underneath just behind the flaps? I'm thinking of taking a saw to this area, but I'm open to new ideas.
I'm about to tackle two, make that three issues underneath. First, open the cowl flaps, two build and install a radiator along the full length of the cowl, and third fill the gap at the rear cockpit at the flap area which is huge.
Looking at your image, I see the area you are referring to. The kit just doesn't have that open like that. I'm not sure that is an area I want to tackle. I'll add the slots, but the long opening at the rear may be too much to do, for me anyway.
What I am planning on doing:
1. open flaps, easy peasy
2. build radiator, copy design from Hasegawa kit and fit it the this kit, make the bulk head solid top to bottom at the front of the wings inside the fuselage, then the air deflector on the rear of radiator will meet this and close the opening. The front edge of the wing has a small bit already, I'll just put sheet stock on that and fill the firewall area, just to close it up. This will have to be inside the fuselage first and then dropped down onto the wing.
3. with the flaps closed the gap at the fuselage is about 1/8 in. Ill fill that with sheet stock and that will also push the wing root a tad higher, making a better fit at the wing root. If you look at this area closely, you will see it is not flat but has a peak if you will. It will take filling it with sheet stock either bent up or built like the peak of a roof so it follows the belly's shape. A piece about 3/16 in wide and from side to side with that peak will fill that up nicely.
I also found that with the dyhedral set all I have to do to close the wing roots tight is widen the fuselage at the flap area. I'll leave the cowl set at the kit size as this fits the front of th wing roots great. BUT, the reat has a gap. With the tail also set at the kit size at the rear wheel, you can just open the fuselage at the flaps about >1/8 in and then taper it closed at the rear wheel opening. This closes the wing root gap and is far easier to do than fiddle with the roots themselves.
All the time I spent working on getting the wings just right has paid off as it allows me to keep everything else at the stock kit attachment points. Otherwise I would have had a ton of work in other areas that would have actually been more difficult.
In the photos, the fuselage is just resing there. The gaps close totally when you hold it in place. It's a nearly perfect fit front to back.
On the bench: 1/48 Monogram P-40B--Early Pearl Harbor (80+% done)
1/48 Tamiya Bristol Beaufighter, Night fighter
2x Monogram 1/48 Kingfishers with cockpits and floats, full kit overhauls each