Removing distributor...

Not if put back exactly where it was.

 

Sure, you can just mark the distributor body in relationship to the mount and the position of the ignition rotor. Take it off, and then put it back in the exact same spot.
But this is not the ideal method. I personally would remove the flywheel inspection cover. Clean the flywheel, paint the PM1/4 (TDC mark) and the ignition timing marks for initial advance, and full advance. Run the motor and check the timing at idle and full advance. Then set the motor at initial timing mark AF for 1/4 bank. If the car timed properly with the timing light, you should be able to use a ohm/continuity meter to check when points open. Loosen the distributor and rotate it, and the points should open right where you marked the body of the distributor. Now you can remove the distributor. When you reinstall it, just mount it up and rotate the body until the points just open, and you should be really close when you check the initial advance with the timing light. I would recommend painting the marks for the 5/8 distributor and checking the timing at the same time.

 

Fuel and air blowing back through the carbs sounds like a cam timing issue. Have you started with the basics? Checking compression and cylinder leakage, followed by checking valve clearances and cam timing. Then check ignition system function/timing. Carburetor adjustment comes last.

 

If the cam belt tension was not set correctly, the belts could have jumped a tooth or a couple of teeth. It is important that you don't run/drive the car until you can make sure that the cam timing and belt tension is ok. If it jumps more than one or two teeth, the valves will contact the pistons with disastrous results (ie: bent/broken valves/valve guides, damaged pistons. There are marks on the outside of the cams that you can install a factory tool on at TDC in order to verify cam timing. A good Ferrari repair shop should have this tool. It allows you to make a quick check to see if the cam timing is off.