Use a cam with 260° - 275° duration and maybe throw in some Rhoades lifters for extra low end grunt and vacuum along with more MPG... Shoot for a little bigger than a stock 340 cam... Crane makes a good economy and power cam...
Install larger valves in the stock 318 heads and maybe some porting...
OR if you want to run 340/360 heads, have 2.02" intake valves installed, but the larger port heads also have larger combustion chambers.. If you want to get the most from them, you should install 10.5 compression 318 pistons and with the larger combustion chambers in the 340/360 heads you should end up around 9.2 compression which will run good on pump gas... Stock 318 pistons with 340/360 heads will get you around 7.5 compression, but you will still get benefit from the larger easier breathing ports... You may also consider porting the 340/360 heads a little...
Run a dual plane intake, like the wiand stealth intake:
CHRYSLER 5.9L/360 Mopar small block LA Weiand Stealth Intake Manifolds
Then top it off with a Holley 80457 600 vacuum secondary carb and 5/16" thick split carb gasket:
Here's the Holley 80457 carb, it's calibrated for a late 60's engine and runs pretty good... I've run them for decades on daily drivers and had great performance and economy with them...
Holley 4160 Aluminum Street Carburetor
I like this gasket as it keeps the plenum split intake all the way to the carb which makes the signal nice and strong...
Edelbrock Heat Insulator Gasket
Also upgrade to a good double roller timing chain, I've run this Edelbrock true roller double roller for years and they hold up well and will easily go 150,000 + miles on a street driven car.... This one is one step above the stock 340 double roller timing chain...
https://www.manciniracing.com/edelbrock4.html