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I am doing something similar, as I needed a good fuel economy vehicle to get to work and back, but I wanted more capable machine both on-road and off-road. I started with a Chevy 1991Regular Cab K2500LD 5.7L V8 that I already had that always got 10-12 MPG, and was able to get it up to 18 MPG average. That involved (in order of magnitude) a transmission swap (auto->manual), driving style, tire swap, air dam, fluid changes, etcetera. I eventually realized I would be better off starting with something that fit my needs better, so I got a 2005 Toyota Tacoma Access Cab 2.7L 5-Speed 4x4. Right out of the gate I instantly got 26+ MPG. The Tacoma is no slouch in the off-road world either.
I am not trying to dissuade you from “running what you brung” as that is always a good idea, just make sure you are going to be happy with the finished result before you put lots of time and effort into it. I have been there done that and it’s not fun. Also don’t discount the capabilities of your stock vehicle, get some decent recovery gear and a buddy and go see what your vehicle can do before you spend a bunch of money and time improving its off-road abilities. If you don’t already, look on expeditionportal, plenty of people on there doing incredible things with almost stock vehicles. Too many sites telling you to never even leave the pavement unless you are locked, lifted, 37s, and one tons.
I have had many of the same ideas you have had with regards to underbody armor, but the trick is to make it strong enough to take the abuse, but light enough not to kill FE and acceleration. No point in the V8 if you have so much weight bogging it down that it drives like an I4. My idea is to use selective steel reinforcements in key areas, and to span the majority using aluminum and HDPE. Look up what many of the tube buggys have done, weight is a big deal to them and there are lessons to be learned on design and construction. Use your armor sparingly, as a protection, not as a first resort to just power through an obstacle.
Mounting is always going to be an issue. Also, don’t create an oven, you need to realize the exhaust is like a giant heating element and if you trap the exhaust up inside a belly pan it turns into an oven. (An oven with your gas tank inside.) Plus the heat generated by your V8 exhaust is orders of magnitude more than generated by a hypermiled Honda Civic. I don’t suggest running the belly pan under your exhaust, it is possible but it takes some serious head scratching.
For the approach angle/air dam dilemma, one method is to use something flexible, like conveyor belt, as the dam so that it can take some abuse. Another method is to use something that is quickly removable. Also, you could do both, which is my preferred solution.
Tires are going to be a compromise. Either get all terrains, or run two sets one on-road only and one off-road only. At least that way you don’t wear out your offroad tires pounding the pavement. Also, I have found siped road tires do much better on slick roads and in light snow than deeply lugged off-road tires with minimal or no sipes.
The top part of your boat tail looks good, but I think you have the sides and bottom too steep. Consider making a small solidly mounted boat tail (that doesn’t come out past the bumper) which can’t affect offroad abilities and would have room for your spare, and then making a removable tail extension.
The burning question in the room: What fuel economy are you getting right now? If you don’t have one, get a ultragauge or other gauge and start learning your car’s patterns and rhythms. Miles from pavement, you need to know when something is wrong well before the car throws an check engine light at you.
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