I had a 94 and a 95, both with the 350. Could not tell you about the difference with the 305 but the 350 was getting _almost_ the same mileage than my previous S15 with a 4.3l
I know about something though, the '95, for some obscure reason did not have the locking rear diff (posi-traction) and it made a hell of a difference in snow driving, traction AND braking was poorer so I suggest that you check if it has this option. I don't remember the RPO code as it appears in the glove compartment but make a search on that, it is worth it.
Good trucks by the way, but then again, I'm biased ¦¬D