not really enuff info to tell hp but figure 200-300 range depending
on compression/distributer setup/carb/exhuast.
also with 350 heads you really need to find out what year heads
and block are for headgaskets. i think 76 was the change year
they will have less surface area and larger coolant passages for starters
you will need to get the right gaskets or you will run into a overheat
problem and they can /or will leak antifreeze into engine
mpg will depend on rear gear/carb/how much your foot is into it
17 in the city is asking alot but maybe.
dont use the 307 intake,your wasting your time and effort
it will choke the engine and it will run like a 307 and probably run lean
dont use the 307 heads either. you will see the intake port difference
once you pull intake from 307,and compare to 350 ports.
65 special v6 daily driver-this needs to go faster possibly a 380 stroker.
82 gn clone 455 12.50's on pump, soon trishield 464
72 skylark convert#2 ,N-25