the v-6 jimmies had a love affair with the gas station. 3 1/2 to 4 mpg was not uncommon for road tractor, and about 9-10 for pickups. pretty reliable as long as you spent plenty of time under the hood. they are a long stroke engine, so will have plenty of torque, but had some flaws that could have made much more actual horsepower.
updating the carb is a good idea, and so is a hotter coil. dual exhaust is pretty much mandatory for decent performance with low restriction mufflers (not necessarily loud.)
they were a good workhorse engine, and in my opinion the 401 magnum was the best one of the series, but the milage was alot of the killer of the engines as for as g.m. was concerned. improvements in diesal technology with better milage was the undoing of the v-6, although the same series of big v-6's g.m. did try with the toro-flo diesal, which was a converted gas engine.
it was an abortion, and g.m. only made them long enough to realize they were a boat anchor, mostly due to the crappy fuel system. that plan was abandoned in short order. green diesals beat 'em out, and brown diesals were better.
