Personally I did numerous chain conversions without removing the Left Cylinder head and I never had a problem installing the distributor drive gear. I do not think the work instructions mention anything about removing the left cylinder head for this repair, to be certain I will review my info.
IMHO-There are many different things that can, and do go wrong. Besides being experienced to address rounded off allen bolts, broken bolts, or even seized bolts you will need to be adequately equipped with special purpose tools. The better quality tools to address this job are mainly German made and they will add up quick. You may want to seriously consider investing in the right repair facility to do this job for you.