Chris,
Since you have the assembly apart, I would suggest that you lighten the flywheel 'some'. The same machine shop that drills the flywheel for the new bolt pattern should be able to do this. Only take weight out of the periphery of the flywheel, try to leave the area around the center hub alone, as that is where the strength is needed. Likewise, the A112 (or 903) used 8 x 1.25mm flywheel bolts and a 10mm 'dowel'. You will have to make sure that the 600 flywheel you are using is set up for both. Do not leave out the dowel, as you do not want to rely on the bolts alone for location of the flywheel once it is balanced.
Again, check to make sure that there is a step up of .5mm from the outer ring, where the pressure plate mounts, to the friction surface area. Make sure the friction surface area is clean and flat.
If the ring gear is some what worn, and you want to replace it, now is the time. I have this in stock if you cannot find locally (29 Euro).
Finally, after all of this has been done, you should get it balanced with the crankshaft from the new motor. Have them balance the crankshaft first, then with the flywheel, and finally with the pressure plate attached. In this way you can change the pressure plate later without having to rebalance the other assemblies. The better the balance job, the smoother the motor.
Hope this helps and good luck.
Regards,
Paul
Scuderia Topolino
USA