I've replaced the exhaust manifolds on S-52 & S-54 M Rdstrs/Coupes, and on the Z3 2.8 (M-54) Coupe.
It's doable, but patience and a full toolbox* is required.
* you'll need every configuration of an 11mm socket, and 11mm wrench that you can imagine, and an assortment of extensions, in BOTH 1/4" & 3/8" drive. There are even instances of me making wrenches to fit the job. This is just the manifold to head interface, the rest of the job is pretty straight forward. On some cars, I am left wondering if it wouldn't have been quicker, let alone easier, to have removed the engine...
1957 Austin-Healey 100/6 Wine Red
1961 McCulloch R1 go-kart Screaming Yellow
1999///M Rdstr Cosmos Black Eurosport Twinscrew S/C
1999///M Coupe Estoril Blue Eurosport Twinscrew S/C
2001///M Rdstr Steel Gray
2011 X5 35i Sport Deep Sea Blue/Cinnamon