Do a proper tuneup. Replace the plugs, points and condenser, distributor cap and rotor, spark plug wires, and fuel and air filters. OEM AC Delco parts should be fine, there are no magic solutions. Don’t expect much more than 10-11 mpg in city driving though. Summitt Racing or Jegs should be a good source for parts.
I did all this to a 69 Buick Skylark with a 350 2 Barrel Rochester and a Powerglide 2 speed transmission. City mileage increased from 8 mpg to 11 mpg. Adding a straight-thru muffler was good for ~1 mpg. Highway mpg was around 15 at 55-60 mph.
A leaking radio? Perhaps you meant the radiator. Where is it leaking, radiator cap, side tanks or fins? Check and replace the radiator and heating hoses if necessary.
Ed B.