Before you go changing things try some simple things first. Since the engine probably gets a lot of idle time and only moderate rev rates it's doubtful swapping to a 2V carb. will make any real difference. Being you don't really use the truck on the road the carb. only operates with the primary circuit anyway so changing to a 2V could be a wasted expense.Besides which the intake manifolds are different which adds to the expense.
A good overhaul of the carb. and set to spec. could be all it's needs(?). The 4150 can be finicky and proper adjustment is essential.
None of the IH SV8's were know for their fuel economy,afterall they were designed 50 years ago when things like that weren't really important,gas was so cheap nobody really cared either.
A complete tune-up with everything set to spec. and "fine tuned" can make a big difference too. I always used a timing light,dwell meter and vacuum gauge to tweak things. Try that before going for an Ignitor which may or may not make any big difference in how the engines performs.
A slightly hotter spark plug may also help,cutting down on plug fouling and incomplete combustion.
Back in my dealer days the parent company ran a fleet of school buses (about 800). The shop supervisor was a hands on guy and rebuilt all the engines himself,I never met anyone who knew more about the SV8's than him,he was genuinely a "guru". He would only use Champion UJ-8 plugs,which was by spec. for IH industrial engines. They are just a bit hotter than the truck spec. RJ-6 but not hot enough to burn pistons. The light line trucks and Scouts spec. plug is the RJ-12YC which is rated as too hot for medium duty trucks. The UJ-8 falls in-between the RJ-6 & RJ-12YC. I asked him one day why he used that plug and that was the answer.
Here's some links for carb. manuals,just so you have them;
http://www.carburetor-parts.com/4150-4160_ep_338.htmlhttp://www.carbkitsource.com/carbs/tech ... dular.htmlA number of guys here have used Mike's Carburetor Parts (1st link) and were very satisfied.
Btw, Did you paint that thing? Looks much better.