Looking at the specs that looks like a great tractor. I have an
L4400 (which does not come with a cab) and it has comparable specs. I don't remember for sure but I'd guess the 3940 probably has better loader and 3 pt hitch specs. But, I've been able to do a lot with my 4400 and it is the perfect size for me.
However, you mention things like 1500 pound rocks, etc and that makes me wonder if a larger tractor would better for you. That may depend on your budget limitations. But, if I were buying right now and if I had the money, I'd be seriously considering the
M5040. You can get it with a cab and it is more of a true utility tractor than a CUT. It does not come with HST (which I don't care for anyway).
If you have to have HST then you might want to look at an L4x40 series tractor.
Believe me, I'm not trying to talk you into a bigger tractor. The 3940 will probably be more than enough and my 4400 has been perfect for me. 'Just right' is better than too big or too small. But if you have the money and can get more muscle in a tractor with similar dimensions, then I recommend going for it. You don't want a tractor that is too big to get around in your work environment (I think a 5040 would be absolute max for me) but you don't want a tractor that is too weak either.
You'll get more advice here and also talk to your dealer about other folks in your area are using for similar jobs.
The only absolute advice I'd give you is to get 4wd. No question.