I'll second the insulation idea - and while you're at it, do the rest of the truck too.
My frame is boxed to a bit behind the front spring rear mount, seems to be working fine.
Make sure you tie the boxing into the steering box to reinforce that area, common crack area.
At this time my truck has a 6" front lift from National, and that's as high as it's going to go. Still has the rear springs from Superlift, but eventually those will go away too, in favor of Nationals. I haven't had an issue with National, but that's because I live close to them, and when I had my springs made I was right there all through it. Every vehicle is different, and they had the springs on and off three times before they were happy with it. I think one of the major issues with doing something like that remotely is every vehicle is different and it's impossible to get perfect results without having the truck there. They can get only so close. But if I were you, I'd check into local spring shops. It's just leaf springs, not rocket science, and if you'd like I can show you what my front springs look like to give you a better idea as to leaf count, wraps, etc.
I've got the NV4500, works OK, would like another gear though, hehe. But it works, and I have no plans to replace it.
Regarding the torque lockup on the Detroit, you can get used to it. Just has different mannerisms, but I agree that something selectable would be ideal. I have front and rear detroits, but I don't live in an area that's got snow, my traction issues are all off road ones.