I put a more "mild" 460 in one of the 78 Broncos I had. Good rebuild, mild "RV" cam, dual plane intake, good ignition and free flowing exhaust and I KNOW it got better fuel economy than the stock 351M. The primaries in the 4 barrel are smaller than the stock 2 barrel. The 460 will have more usable horsepower and torque than the 351M and at a lower RPM. Keep in mind that with the free flowing exhaust...you'll want to hear it....then your economy will go down!!
If you want fuel injection, there are a number of after market companies that offer a complete system for engines that were never fuel injected. They pretty much come with everything you'll need right out of the box...fuel pump ECU, lines, harness, sensors and throttle body. The ECU's that come with these systems are user friendly, self learning and and some have different settings for different driving conditions...towing, economy or extreme/race mode. They will most likely be a lot easier to install and surely more inexpensive than trying to retrofit a system that was not designed for your truck.
I really don't think you'll be happy with the 5.0. Definitely keep the 205 transfer case. The C6 is a good transmission...very tough but probably has the worse pump power loss ratio of automatic transmissions out there. The good news is it is probably the cheapest and easiest to fix with a high performance pump upgrade.
Look at some of the performance sites....and price different EFI systems. You will surely get "sticker shock" at first, but will most likely be happier in the end.

 

the ZF is great, its basicly a NP435 with over drive. those 435's with their granny low first gear are almost bullet proof.

 

Ya i pretty well got it all figured now i picked up that F150 with the ZF 5speed for $50 SO I will pull the tranny and clutch assembly out of that truck and part the rest out rearend, Tcase and motor. And im going to check out two kinda beat up 1988 broncos with 5.8s for $200 a peace today there both auto c6 but more parts to sell. Or mabey with the spare parts ill build my self a 5.0 bush buggy or something lol

 

Im not to worried about figuring out how to hook it all up. It all seems farly simple. Buying some $1200 dolar EFI system seems like much more that i wana spend when i can get a good old ford set up for next to nothing. One thing import starting to think now is header for the 5.8 do i just order ones for the 88 bronco will the fit the 78

 

you wont know regarding headers until you have the motor in place.

 

Along with kemical, the body shapes and engine bay room is a bit different.

 

You might want to get the headers locally, so you can return them if they don't fit.

 

Ya buy em localy that's what ill do. Looked at the two bronco both 5.0 though, Dam the offerd them both to me for $300! But i can't have to many old fords in drive way hell of deal though http://kamloops.kijiji.ca/c-cars-vehicles-auto-parts-tires-other-parts-accessories-1988-Ford-Bronco-SUV-W0QQAdIdZ483227719