Hello!
Squeezing people in the equipment is almost always hard in scale. One thing is all the walls are usually overscale in order for the model to remain structurally stable. The other thing is, like Stik just wrote, all the cushions are "unweighted", and the poses of the figs need adjusting too. And every millimeter you leave out give you real live inches, and two or three inches most of the time make a difference between a "spacy" and a "crowded" seat.
My advice would be to always "frankenstein" the figs to the space they are to occupy. Instead of cutting a piece of helmet off, it would be much better to cot a piece of their behind off, to show weighted cushions - and frankly most of the butts also deform some under load. In a pinch it's best to cut a millimeter or two from the lower torso - a slice along the belt usually can be made very easily. Makes the fig instantly shorter. Cutting a fig at the jonts and wiring them also helps a lot to exactly pose the figures.
Recently I'm working on something like that, but in 1:72:
Notice how big the gunner is? And the pilot's helmet was first three millimeters above the canopy surface, but I managed to squeeze him in by cutting down his pants, shortening the torso, shortening the neck and fitting all the parts precisely.
In 1:35 I've done this, too. Here's what I got:
How I got there:
More on that here:
http://www.vietnam.net.pl/M42mod6en.htm
I hope it helps - good luck with your builds and have a nice day
Paweł