The restrictions in a 305 are it's awful bore size coupled with it's diminutive size with low compression, and you severely need compression @ 7,000'.
Not the heads which are normally just fine to boost compression on a 350, (unless you have the worst versions).
The cam is sometimes smaller than a 350 cam specs, but not always, and the 305 needs a very small cam to help to not lose compression - especially in a truck! But a BETTER cam always helps, Isky and Comp both make some very good tiny cams ( most truck 350's need these also).(A cam is down the list about 4th)
Add a K&N air filter to your stock airbox, but it will be expensive
Long tube headers always bring extra power and a mileage boost if you free up the remainder of the exhaust. A higher flowing single pipe system can be better than duals when done correctly with a hi-flow Y-pipe and low restriction muffler, (A mandrel bent 2.5" system). This would be priority one!
Set a new stock 350 installed with better exhaust and you'd likely gain mileage and definitely feel like double the power. And I'd use new/good 305 heads on that 350, 1.84 valves and all!!
