My son wants to do the same to his 73 Scamp. What you need to do is measure. Get the car on four jack stands resting on the suspension, remove all four wheels. Use a straight edge to measure vertically from the drum or disc face into the wheel well approximately the height of the tires you plan to use, then measure in and out to see what the backspace needs to be with 7 or 8 inch wheels. Allow at least an inch on all sides for clearance and don't forget about suspension jounce and rebound. You may have to push the inner wheel wells in/out and or roll the wheel opening lip.
Do this for all four wheels so as to take into account any stack up of tolerances. In other words what fits on one side might not fit on the other.
As far as the front wheels go be prepared to either make a mock up of the size tire and wheel you plan on to see that is clears while steering and suspension jounces and rebounds, or trimming the fenders for clearance.
