72 slant six upgrade

I agree with Rusty,driving slants from the 60's to present day one thing they were never known for was for fuel mileage. Most averaged 14 to 16 on a good day.Flat land areas may have gotten better.Rear axle ratio makes a difference.Talking A bodies early slant cars with 170 engines usually had 3.23 gears.225's on the average had 2.93.Depending on the terrain you drive you need to gear for those conditions. For me out west roads are mainly flat with speed limits in the 65-75 range .I've gone with 2.76 with great results being a max of 20 mpg for the roads traveled around here.Any engine will benefit from better breathing in and out.
So the question is what is the condition of your engine.? First check compression, if the cylinders vary more than 25 psi you may be chasing your tail looking for performance and better fuel economy.
If you find it to be healthy,Then a intake and exhaust change will help you to reach your goal.Along with a good tune including valve adjustment. Have fun with your project and thanks for wanting to work your slant.