I’ve built several AMT/AMTech P-40s, and I think I have them pretty well figured out. The gaps at the wing roots can be minimized by adding a fuselage spreader just forward of the cockpit. This pretty much will close the gaps, and leave some room for setting dihedral. You can glue the upper wings to the fuselage, and then glue the lower wing on, but I find this often results in either too much dihedral, or very poorly aligned gun ports.
I used some resin bits on the interior to dress it up a bit. It’s not a great kit, but it’s not bad either. Basically, if you want a P-40F/L- this is it.