a turbo definitely won't give you any better mileage. you're increasing the volumetric efficiency of the engine, and to maintain a safe A/R ratio, you'll be feeding it even more fuel. on your V8, it will probably cut 2-3mpg off your mileage, even when driving it easy.
but to install a turbo setup - you'd need some custom exhaust manifold work, either custom headers, or to pipe the exhaust flow to a turbo...probably be easiest to do a remote mount.
then you'd need another set of piping to run the intake air into the inlet of the turbo, and from the turbo outlet to the throttle body. this stuff is generally 2.5" to 3" thin wall metal tubing. it would also be ideal to run an intercooler in front of the radiator, which would more than double the tubing requirements.
then you'd need to do some tuning to the ECU so it would maintain proper A/F ratio across the RPM range...that means some dyno time at a shop, and you'd probably need to buy a piggyback ECU, but i don't know how smart the 5.9L ECU's are.
All that has been done...Dodge 5.9L R/T Magnum's were pretty hot trucks to build up back in that time period. KRC Performance used to be a big player in that area. They had a ZJ running 11's in the 1/4 mile, and they had a twin turbo 5.9L Magnum R/T pushing like 800hp or something ridiculous.