you might also want to check the distance between the holes for the control arms. keeping these within the same ballpark as the 1200 crossmember is important for steering geometry. As the 310 is a much wider bodied car, I suspect there will be a significant difference to the 1200 x-member.

I might be missing something really obvious (it wouldn't be the first time!), but adapting a x-member from another non L series powered car just seems like making extra work for no benefit. I think its easiest to simply make mounts for the 1200 x-member, and slight mods to the sump to clear x-member