Bob, first thing is that your bees ought not to be needing fondant at this time of year.
Second thing is that with massive Dadant hives, you should have LOTS of stores in the brood box, and ought not to be needing fondant even at the very end of the winter!
Some would routinely feed fondant whether or not the bees need it.
Which seems wrong to me. The long-term goal is to have the brood box mainly clear for brood in the spring - excess unused stores is a principal reason for early swarming!
If feeding fondant, the two options are to put it on the topbars (either a thin layer, not much in there, or a fat slab in an eke) where the bees can easily access it, or above the crownboard where the bees access is much harder when the weather is really cold.
My personal preference is the eke (if fondant is needed, it needs to be both plentiful and accessible) with an (of course) no-hole crownboard with insulation above.
But they must have had a really pathetic autumn's forage for them to be needing fondant before Christmas.
So I think the need for fondant (or frame shuffling) should be assessed at Oxalic time, when the hive is open anyway. They really shouldn't need it before then. However, if your hives really are light right now ...