Was the Falcon originally designed with strapped on in mind?
I think there is a version of LM5 with LM7 strap-ons' as boosters.
http://www.americaspace.org/?p=22881
"The Long March 5 alone has 6 configurations."
In any case the central LM5 core could be suitably strengthened if 2 or more LM5 strap-on's are added.