Sorry I didn't understand well
How do I have to do it ?
Sorry for my silly question but I'm not a mechanic
Thanks
Good luck tomorrow hope it is all ok. Just a quick hint before you pull the distributor, you might want to turn the motor over by hand and get the rotor in the distributor to point at #1 cylinder. that way when you drop in the new one you can just line the new rotor up to #1 on the new distributor and it should be very close when you drop it in.
Under the distributor cap is a 'rotor'. Each cylinder has a spark plug wire attached to the distributor cap. When it is that cylinders turn to 'fire' the rotor will be point to it. The 'firing order' and even the designation of which cylinder is #1 varies even amonth other v8 engines or even other ford engines. The picture about shows the firing order of the 351C.
It is very important to note that YOUR #1 Cyl spark plug post on the distributor cap MAY NOT be at the same position as shown in the picture. You will want to trace the spark plug wire from cylinder number one (front passenger side) to its post on the distibutor cap. This will be the #1. Mark on the outside of the distriubtor where #1 is. It has to be marked in a way you can use it with the distibutor cap off. Then remove the distributor cap. You will want to rotate the engine until the rotor is pointing to the mark you made. This can be done by 'bumping' the starter (turn the key to the start position quickly) but it is almost impossible to line up so most people do it 'by hand'.
To do it by hand you can either take a large socket and socket wrench to the large nut on the front of the motor that is inside the large pully and turn the engine over (rotate) or often times if you just grab the large pully with your hands the engine can be rotated. Watch your fingers.
As you turn the engine over by hand you will meet some resistance as each piston takes a turn on its compression stroke. Just stop for a few seconds, you will hear the pressure being released and it will be easy to turn again. If you accidently 'pass' your mark it is okay to rotate the engine in the opposite direction a little.
Let us know if this isn't clear. I see you are in Italy and I do not know how good your english is. Some phrases don't translate well, like 'turn over the engine' is actually just rotating the moving assembly.