I'm using the 606 turbo engine which has an ally sump. The engine is inclined naturally so doesn't have the same height issues. The down tube is being routed in front of the main subframe Xmember which is where the original went in any case. My car was a Euro 280SL in it's past life . That will require the turbo to be re-mounted back-to-front with the Xhaust outlet facing the rad but that is a minor job.
BTW, I'm using the same type rubber feet as you but uprating to the petrol 500 V8 version. I did a deflection test on the original 280 mounts and compared that to the hydraulic ones that came from the W210 donor car. The difference was 1.5mm more for the 280 ones. Almost insignificant but I'm going to the V8 ones which have a harder compound (I'm reliably informed). Interesting that you filled yours with urethane. I'm now looking at sourcing some of the same in the UK. My original ally engine feet are way out so will be fabricating new ones to suit the mounts using 10mm steel.

There's a member of the UK Mercedes-Owners-Club who has the same engine conversion as you in an R107. Unfortunately, he's not on the Club forum so I can't ask him questions.