If you are determined to do this, then here's the skinny. Picture on left shows a flywheel for MGA (or Z-Magnette) or early MGB with 3-main bearing crankshaft, 1953-1964, 120 tooth ring gear. Picture on right shows a flywheel for MGB with 5-main bearing crankshaft, 1968-1980, 127 tooth ring gear. If you want to mate a 5-main bearing engine to a pre-1968 gearbox, you would need a flywheel with the pilot diameter and bolt pattern pictured on right and the smaller ring gear in picture on left (like flywheel from 1965-1967 MGB).

The picture on left is a Fidanza 126991 aluminum flywheel with standard steel ring gear and a steel insert for the friction surface. There is some confusion about the configuration, because most web sites have the wrong picture posted, and also the wrong list of applications (although most agree on 1955-1959 MGA). Even the Fidanza web site has it wrong (and the Fidanza people are sometimes not much help). This picture came from the Summit Racing web site, and they seem to have it right. There are always six bolts in same locations, but the alignment pins vary for different applications. Two pins in locations "A" are for the Z-Magnette or MGA clutch. Three pins in locations "B" (close to the bolts) are for the MGB clutch.

Except for number of clutch alignment pins (2 or 3), original flywheels are physically interchangeable for MG ZA and ZB Magnette 1953-1958, MGA 1955-1962, and MGB 1962-1964. In 1965 MGB changed to 5-main bearing engine with different pilot diameter and bolt pattern. In 1968 MGB changed from 120 tooth to 127 tooth ring gear (larger diameter flywheel), but retained the same clutch parts. Z-Magnette and MGA had 28-lb flywheel with 2 pins 1953-1961, then 20-lb 1961-1962. The same 20-lb flywheel carried over to MGB 1962-1964 (3-main bearing engines) but using 3-alignment pins. Fidanza is keen about this, so their flywheel has five holes for alignment pins so it can be used with either MGA or MGB clutch, and all Z-Magnette, MGA, MGB 1953-1964.

Fidanza part numbers:
126991 is for all 3-main motors.
126681 is for early 5-main with 3-synchro gearbox.
126801 is for later 5-main with 4-synchro gearbox.

126681 can be used when installing a 5-main MGB motor in the MGA with any 3-synchro gearbox.

You will want to check and maybe have the new flywheel balanced. Apparently the Fidanza flywheels are not factory balanced, and they make mention of this on their web site. Some people use them as-is out of the box and may get away with it, but some may be out of balance by as much as 15gms.