Ok, my two cents. Keep in mind that I am a big Full Size Jeeper with a bad ford addiction building my wife's chevy...Ive had some experience all the way around that seems to lend itself to this particular question.
For the carb, I'm assuming that you are still running stock heads on your 400, and are looking for some sort of torque building combo that will serve you well in the mud and playing around on trails. You also are tied to a TH400, and an NP203, so here is what I would look at and why:
Heads: Keep your stockers, but have them port matched to your intake and exhaust. The stock heads actually flow just fine for your intended purpose, and since you could (read should) have them off anyways while installing your new cam, it doesn't take much to use a dremel and make things a little smoother and keep your fuel/air speed up coming through your induction.
Cam: I have had great experience with CompCams and there Extreme Energy 4x4 cams, though the Magnum series are also a massive improvement over stock. I recommend that you stick to around the 256-268 range, as that will cover your RPM range well. It really depends on how your drive, if you are a little heavy footed, don't be afraid to go with the 268H, remember that you have a 1200-1400 stall speed stock anyways, so you can afford a little higher rpm range and work fine for you. If you tend to Crawl along and only hit the gas when you really need it, 260H or the 256H are more inline with your driving style.
Intake: Stock works fine, but if you can wrangle up an Edelbrock Performer series for cheap, it's worth it. Hands down great torque and horsepower builders, flow air very well at low speed and look nice too. Usually can find a good used one for the $100 range. Either way, spend the time to make sure that everything matches up port wise, it really does make a difference.
Carb: Here is the great debate of all time, Holley, Eddy, Q Jet, or some brand no one has ever heard of. Best answer? Depends on your driving style and where you play and what you intend to do. I love the Holley Truck Avenger series, spring loaded floats, outside accessible float bowls, ready change and easy swap jets that were color coded...awesome for blasting up fire roads and log trails, getting a little of camber. Con is that you are constantly adjusting them. Q-Jet, by far the most economical carb I have ever run, even though it is a 750 cfm. Why? Because when I kept my foot out of it I ran on the primaries only, which only flow like 350 cfm, but have massive secondaries for when you bury your foot. They do fine off road but in my bronco didn't seem to like being bounced around as much. Swapping needle seats for altitude and adjusting floats on the side of the trail are much more invasive with this carb. I just put an Eddy 1406 on my Suburban and I gotta say that it's a pretty good compramise between the two so far. Much more tolerant offroad and setting idle speed on them is dirt simple, no gasket below the float is awesome, but doesn't make the same economy as the Q-Jet or have quite the throttle response that the Holley did. For my money with what you seem to want to do, I'd get the Edelbrock if you can get a good deal on one, or save and buy a Truck Avenger 670 cfm. If you are set on a Q-Jet, pm and I can send you mine. I won't need it.
The idea for building good power, especially down low, is that the less the engine has to fight itself and the better it can breath at your intended rpm range the better it will build torque and horsepower. Many combinations will work, but its the one you spend the time developing for how you intend to drive it that works best. Hope that helps and good luck!
