Have you considered running a non-oversize Thomson stem? It'd pass as retro and give you none of the disadvantages of the Syncros system.
My vote was for Thomson for the simple reason that it won't crimp your steerer.
I always thought Syncros was over-rated though, even back in the day.
The Thomson seatpost is probably the most precisely machined seatpost ever made - in terms of guaranteeing the stated size is the actual size, to the 2nd decimal point. (meaningless unless your seat-tube is reamed with the same precision - few are - but if you have an over-reamed seat tube and slightly-less than stated diameter seatpost, it can give "issues".
So I'd go for that too. It's also passable as retro.