I googled "380SL performance mods" and received google's version of a horselaugh...

Did MB ever couple the V8 with a manual transmission in the R107?

Found a firm that offers manual trans conversion for R107:

https://www.theslshop.com/parts-shop/mercedes-benz-sl-107-116-126-v8-automatic-to-six-speed-manual-gearbox.html

$14k US, which seems quite affordable...