I have wanted to slap a five speed behind my 300SEL for a while and even perhaps my W116 280S. I made a post about this a while back.
I have seen many M110 cars with a manual (four and five speed) and some M103s with a five speed too (mainly in W201s).... this includes the ECE R109 280SE/L with a manual.
So, I know the pedal box exists for the swap for the W126... which to me is the biggest part of the mystery. Apparently there were some 3.5L V8 cars with a manual too back in the SLC days. The US 4.2L M116 does not make that much power... when new it was 201hp/221tq.
I say go for it, it would be an interesting project.