Re: xm engine crossmember
if the crossmember bolts to the back, then it is the same as the XL.
I have a set of engine mounts for my XL out of an XP (for a windsor)
Unfortunatley at this point, i cannot do much trial fitting for you, as the XL still has the 200cid in it...i have yet to start this project.
At this point dimension 'B' you seek is irrelevant. Sometimes you need some persuasion to make them go in. i.e a podger...esp if you are dealing with engine in.
The V8 mounts i have for the XL (from an XP) are integral with the crossmember, i.e they share the same mount point and bolts......confuses to me why i asked where your crossmember mounted in hindsight.