If you plan on street driving with a manual valve body I suggest making sure the rest of the drive train is super strong. The added traction will tear up parts with the hard shift that comes from a manual valve body. Or run some rock hard tires. If this is a dirt only truck than the tires will spin when shifting. My own experience with manual valve bodies is that while fun to drive. They are tiresome in traffic and hard on internal parts. Look for a valve body that will shift normally in drive and has manual mode when shifted. This will help protect the transmission and your wallet if street driving is part of the plan.
I got this motor and transmission for really cheap and was just going to use them to mock up mounts, then I read this article on the 5.3L http://www.trucktrend.com/how-to/engine/1208tr-5-3l-bow-tie-builds-mild-to-wild/ and now I am on the fence if I want to build mine this way or go with the 6.0L. As for the 400 transmission, again I got it really cheap and couldn't pass up the deal. Although that 6 speed paddle shifter does sound pretty badass for a desert truck, lol. I've got plenty of time to figure it out cause I'm still doing fab work to the truck.The 6.0 is right about there with 377 inches. The 4.8 and 5.3 are limited in bore material. Start with a bigger engine you will be time and money ahead. You will also gain durability by keeping more material in the bores. 6.2 gets you right about there. Swap the cam and springs throw a tune in it and you can achieve your power goal pretty easy and with a ton of reliability.