Here is some info from a fellow club member...
Excerpt from Ron Bishop, "Rebuilding the Ford Flathead," which has appeared in installments in Skinned Knuckles
There's one thing certain about breaking in a rebuilt motor; if you ask a dozen different people what's the proper way to do it, you'll get a dozen different answers. Everyone seems to have a secret formula for breaking in a motor, and he's convinced that his is the only right way to do it. There seems to be one common element in all of them; everyone will agree that the most important factor in breaking in any engine is the seating of the piston rings.
Getting a good sealing surface between the rings and the cylinder wall is what determines whether or not the engine will burn oil and also whether or not is will make horsepower. The best way to establish a good ring seal is by running the engine at varying rpm for the first few hundred miles. Be sure to spend some time in deceleration as well as acceleration during the break in. The ring seal is governed by the gas pressure exerted on the rings, and that pressure is determined by engine speed. Running at a constant speed will generate a balanced pressure and may lead to polishing of the cylinder walls. Therefore, continually varying the speed during break in is of importance. The acceleration/deceleration cycle is necessary because this will load both sides of the rings against the walls.
As soon as the motor is started it should be set at high idle which will allow oil to circulate through the engine. Oil pressure is important, so keep an eye on the oil pressure gauge. Also keep a close watch on the engine temperature gauge to avoid any chance of overheating which could distort the internal components at this critical time in the engine's new life. After the engine has reached operating temperature and runs for 20 or 30 minutes, recheck the ignition timing and adjust the carburetor for correct idle speed. Shut off the engine and retorque the head and manifold bolts. Check oil and coolant levels.
Don't take any long trips and don't run the engine at extreme rpm. After the first couple of hundred miles, change the oil and oil filter. It's also a good idea to spend some time checking the engine compartment for loose belts, hoses, electrical connections, and so on. Once the 200 mile mark is reached, you may go ahead and run the engine the way you plan to drive it. If it's not broke in by the time you reach the 500 mile mark, it probably never will be.