1993 chevy silverado backfire?

[Rusty Tonka]

the truck has been gtting a backfire for months most of the time at highway speeds and almost all the time if i use overdrive, if i dont use overdrive, it's nowhere near as bad. plugs, wires, distributer, all new. it has a coolant leak at the intake manifold, that became apparant a little while back, could this be causing the backfire issue?
engine is a 305