just a thought, but people fit mk3 alloys on mk1's all the time, and to the best of my knowledge they were a direct fit (no spigot required)
so if thats the case, it would confirm that the mk3 is a 54.1 centre bore as well. from a brief browse on google most sources seem to say 54.1mm as well
by all means check to confirm though. if this is the case then it makes the rear conversion even easier!!