you probably already know this, but just in case you over looked this ...
with the symmar converted you will need more than 620 to have it focus at infinity ..
don't forget the nodal points are farther out with the symmars than with a traditional lens.
( maybe it is its plasmat-type design? ) i have a 210-370 and don't have enough rail space on my toyo
with it converted, luckily i have another camera with a longer bellows s t r e t c h and it works great.
good luck with your quest !