How long is a piece of string???
If I were you, I would do a test run of your machine. Fill it up with a known quantity of gas, drive a specific distance, see hwat you have left and do the calculation.
Remember, hills, gearing etc will affect the mileage as will idling time, traffic slow downs etc. I would add in a safety factor to account for these things when calculating my mileage.
To venture forth with such a trip based on guesswork is a risky undertaking.
Make sure that you also have plenty of spares to fix the items that may break down en route.
These might include plugs, filters, oil and fuel pump, carb, points, dizzy, coil wires, etc
Has your truck been on the road and if so, how reliable has it been? If you have racked up a couple thousand KMs over the last couple years without any issue, I would think that is a good starting point. If you are rolling out a freshly restored project, take heed. Trust me, in all the excitement it is all too easy to overlook things. All the Best!
