On a closed hydronic system, which is what your cab heater is, getting fluid flow above the top of the rad is tricky.
If it were your home, I would tell you it is not the circulating pump which will move the fluid up hill but rather the system fill pressure. 15 psi of fill pressure will raise water up 32 feet.
I know there are tractors with heater cores in the roof of the cab.
I have not studied how they establish circulation. Others who have such systems will comment.
If I was faced with the situation, I would simply fill the system at the highest point with the rad cap on.
Put a Tee at the highest point and add fluid through the tee.
Dave