Re:
We have 3 different Marin full suspension frames, Wolf Ridge, Wild Cat Trail and a Mount Vision.
The frames are different sizes and each one has a different length shock, 185mm, 200mm (I think) and 165mm, in that order.
If you could fit any old shock to the swing arm but not the frame then move swing arm till there is just 5mm, no more, between end of shock and seat-tube, measure i2i distance for length of shock.