Thanks for the info Chuck - thats 100% consistent with advice from a local MB expert here in Oz. I was expecting the bellhousing to be a different volume (the clutch plate for the V8 is quoted at a larger diameter than for the six, about an inch more from memory) but I figured the bolting pattern would be the same, ie just a case of using the bell housing from the 4spd.

When I mentioned the M110 I was thinking specifically of a 280CE, '77 model which I had seen with a 5spd - perhaps that was one of the two gearbox models you referenced.

Although the manual trannys were officially available for the SL in Oz they are still as scare as Rocking Horse doodoo - I only know of four others and they are all 4s. The only 5 spd box I know of is not for sale so I'm not making much progress on this. I have found a brand new, old stock, 4 spd box which I have been promised for nothing but apart from replacing my still-perfectly-functional 4 spd theres not much advantage in that.

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'71 350SL auto
'73 350SL manual
'98 528i (yeah I know but at least its German)

'71 250CE (sold)