Atlas - Delta Very Heavy

This subject was touched on a few years ago in one of the sci.space groups.
Now that NASA has been given a new direction, maybe its time bring it up
again.
I have been following all the threads on proposed shuttle-C and the need for
a new heavy lift vehicle. And, I keep wondering about a Delta or Atlas with
four additional CBC's instead of two. The question is would it take longer,
and cost more money, to upgrade the Heavy's to Very Heavy than it would to
turn shuttle into shuttle-C, or build a whole new heavy lifter? If it could
be done, how would it compare with shuttle-C in lifting power. My first
thought would be that it probably wouldn't have the muscle of the C but it
might have enough to do the job at a reasonable development cost.
Any thoughts?
Farmer