One of the biggest problems I have always run into is pin mis-alignment. I found that if you sand off the locating pins on the Monogram AH1S the fuse halves line up perfectly and you will only need filler at the tip of the nose and the bottom belly just aft of the aft skid tube. I have built a bunch of Mono Cobras, so I know its not just a fluke on one. My Italari locating pins never fit right. I jus sand them off and then lighly clamp the parts after applying liquid glue. Im not much of a clamper, I find that my rubber bands or tape wick the glue away and down the fueselage, or the Berna assemblers twist parts, or my hobby clamps crush the plastic, you name it. I will use padded worn out clothepins on the tailboom and maybe up on the nose, and some weak clamps around the doghouse if needed, but not much. I just get better results with low clamp pressure. I WILL tape the inside of a fueselage seam if i can get in there, like on any Dragon 1/35 egg or cobra where I can drop in the cockpit.
The other half of this is, with many kits, dont do the whole fueselage at once. Do JUST the tailboom and let it dry over night and then work the rest of the fueselage into place, that way you dont have too muich going on at once. Italari's cobra designs are like this, so flimsy due to the entire upper pylon being a separate piece. And, somtimes one half is longer than the other and you have to work around it as well, maybe taking a slice out to shorten it (tailboom joint is a good candidate, dont remember which kit it was but I had to do that once).
another thing that i often do is back up myparts with glue tabs. the big blackhawk has a caved in belly. I added .040 tabs and chunks of old credit cards to the inside of the fuse seams to help stabilize it and add strength. glue them up on one side and let them dry,and then glue the halves together adding glue to the tabs so the surface area of the bond is increased. Vacformers do it for strenght and it helped on the big 'hawk. I also ran a piece of tube down the TRdrivshaft housing as I have experieced cracking and failures here, having the tubing in there gives about 500% more glue surface to keep that from splitting. maybe its just me but before I started doing that they would split out on me.
HTH
David
Build to please yourself, and don't worry about what others think!
TI 4019 Jolly Roger Squadron, 501st Legion