Since you are stuck with a 4" lens board on the MCRX, something like a 240 f/5.6 El Nikkor or 300 f/5.6 Rodagon cannot be mounted without machining some sort of adapter. There are two issue here. First, the mounting flanges on some of these faster lenses are greater than 4" in diameter. Second, the rear element can be larger in diameter than the opening in the lens stage on the MCRX and may protrude too far. Either way, some sort of custom mount would need to be built and even at that it would be a compromise. Weight is also a consideration. As I've said before, the MCRX chassis was never really designed to hang that much weight that far out from the upright. Adding a fast, heavy, long focal length lens on some sort of adapter is only going to add to the problem.

Longer focal length F/9 optics are easily mounted on the 4" boards, but you have to deal with a dim image for focusing. As for sharpness, any of the modern optics will provide similar performance as long as they provide sufficient negative coverage.