With the 3.2L V6 and the 4.3L V8 being more similar in installation than the I4 it may make it a bit easier but it comes down to the question of being worth it in the end. If you want more power, just get an SLK32. But if one wants to be able to says that they have a V8 powered SLK then get the R171 SLK55.
If one wants a project and runs accross a wrecked CLK430 that can be used as a doner car for the power train, ECU and wiring then it might be slightly better to start with an SLK320 with a manual transmission to put on the 4.3L V8.
It still would not be as powerfull as the SLK32 or the SLK55. The guy with the Crossfire got a deal on a 5.0L SL engine and had to run it in a diagnostic mode or something all of the time so there were complications. He did a nice clean job but in the end, it would always be a project car that is not up to the standards of a factory built model. At least in a Crossfire body the Mercedes moto of "Best or Nothing" doesn't have to apply.
I am sure someone could drop a crate Chevy or Ford V8 in a Benz body and get it to work but who would want it?
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