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You don't need to remove the internal shroud, you just need to sand down its height . It contributes about 1mm to the optical path, and the built-in infinity adustment cannot accommodate without it.
To get infinity focusing, the rear two elements will need to get to paper-thin separation (<0.1mm) from each other (disclaimer: this is from experience with the FD85L on a 1Ds II). So you must test with great care, and use the infinity limiter as the mean to prevent lens collision. And even so, whether you get infinity without mirror shaving may depend on your particular 5D's mirror arrangement. Every little bit counts here, as the 1Ds series get better alt. lenses compatibility (versus the 5D), (may be) by just a hair or two better mirror clearance.
* The mirror of 5D is glued in place by double side foam. Imagine the tolerance involved, it is not hard to understand why some 5Ds get better mirror clearance. I wonder who will get brave enough to poke his 5D mirror off and stick it back a little bit inwards to get better clearance. But for all the work, it may be even better to shave the mirror jut a little bit more .
** For the 24 Asp, search the forum! JimBuchanan has converted one and reported that it cleared the 5D (YMMV). The mirror is closest to the front when it is swinging near the top. OTOH, the last element protrudes only at the very center. so it handily stays out of the path of the mirror. BTW, m-a-x is converting one, and I have one coming, too .