As far as the MII track width - this is from the Fatmans site...
As long as the Lasalle is within these specs I don't see any reason you would have a problem (other than it being Ford based technology...Doh!...
MII "kits" are used in thousands of hot-rods and as someone already pointed out - this is not the MII parts out of a boneyard from a Pinto (nice picture BTW!)
You would, of course, want to use correct suspension components based on the weight of the target application - these being larger sway arm, springs that are made to handle the weight, bigger brakes.
But the other Fatman MII components themselves (a-arms, r&p) should be plenty beefy to handle the LaSalle - a quick phone call or email to one of the Fatmans engineers would really be warranted if you have any real concerns.