According to
this, the 40 wheelbase is shorter than 41-46. S-10 frames are easy to shorter, there is a factory slip-joint that you can take apart, take off some material, and slip the frame together again with the shorter wheelbase. Easy to do. 4 inches is a huge difference and will look terrible if you don't do anything to it, IMO.
I think the 40 cab dimensions are similar to the 46, just a little shorter in the cowl area I believe. I had to notch the inside bottom edges of my front fenders about 1/4"-1/2" on each side for frame clearance, but you can't tell once the running boards are installed as they fill in the gap and make up the difference. That's the hardest area of making the truck fit on the S-10 frame though.