If you plan on doing long distance touring with the bike, the steel Interstate tank is a nice option. The version that came on the MK3 and possibly some 74 Commandos is slimmed down a bit from the earlier gigantic ( 6 imperial gallon?)
The interstate tank is great for long distance. My favorite. I have never needed to switch to the reserve on my 74 yet. From fill to reserve you would have a serious case of numb bum if it was all straight ride time.
On steel roadster tanks I use the 2 hour rule.
Fill....drive and after two hours you are looking for gas station. The final gear has influence in your mileage. The 19 tooth compared to a 21 and Carburetor choice will have a significance for a given highway speed . Amals are not bad, if set correctly.
On Any Sunday...A motorcycle is what ever you want to make it. Turn it on, and you can give yourself a real thrill...