gas mileage in tehory would be better, but if you are working the 350 hard then the mileage may not improve much. what are you getting now? how are you using the truck?

check out http://www.fueleconomy.gov/
i see an 89 350 getting about 14 city, 16 hiway. a dually will probably lose a few mpg to drag and extra weight. 4wd will be worse too. rear gear ration will make a difference as well.
I didn;t see the 3500 or 454 lised.

as to changing it over...to install a carbed 350, crate motor, yo'll spend $5000 or so, and unless you drive gobs of miles you'll never make that back in fuel savings.

if you do the work and go to a junk yard 350, umm used motor, you may be able to get by on $1000. you can do the math and see if that would save you anything at all.

then you mayhave emmissions issues, but the truck may be over the GVWR to need that test.

the motor mounts will be different, no biggie there., the exhuast will differ, not too big a deal.

the trans...bolt pattern may be differernt, the cluthc or TC will be different.

not sure that this would make economic sense...unless you were gonna put the 454 in a t-bucket for weekend fun!