Which M42 adaptor to EF to use?

[badaboom]

Hi my first post. I would like to ask if any one knows which adaptor i should use with my new M42 screw mount Carl Zeiss Flektogon 35MM 2.8 that I am going to use on my canon 5d mark ii. I have heard that it matter which adaptor ring you pick, also I have heard that the mirror hits the lens in infinity is this true?

Thanks allot forum.