The 6.2l V8 engines in the C 63 AMG (W204) and E 63 AMG (W212) are basicaly the same; the one in the W204 is NA, the one in the W212 is TC. The previous CLK (C209) was like the current E Class (C207), it based on the C Class (W203/W204). First the CLK 55 AMG (C209) had a NA 5.5l V8, while the E 55 AMG (W211) had the same engine 5.5l V8 but SC. Then the W211 got the NA 6.2l V8, and the C209, which is in reality is a W203, also got the same NA 6.2l V8 but detuned, just in the same way the C 63 AMG (W204) got a detuned engine version of the E 63 AMG (W212). This means both W204 and W212 can have the same engines. Now the W212 has recieved a TC 5.5l V8, it can 100% fit into the W204, in this case C207. I don't believe there is any R&D or testing to do. Just get the right software, and update the chassis (suspension,...) to AMG parts, which is bolt-on pure. After all that is what BMW did for the 1 Series M. The same goes for the 981, there is no work needed (R&D) to fit in the 3.8l from the 991.