IMHO, you want about 30% head space, so 2 gallons for a 6 gallon batch.
The 90# is about a 5-6 gallon batch. You said the 6.5 gallon bucket is too small. Does it fit with limited headspace, or is it over flowing?
Here is what I would do (since you already have the 6.5 gallon bucket):
1) Defrost what you can in the bucket, and treat with k-meta.
2) Defrost the rest in stainless steel pot (or whatever you have) and treat with k-meta.
3) Remove 1 to 2 gallons of juice (no solids) to glass jugs, or whatever (steel pot), and store in fridge. You want about 2 gallons of headspace in the bucket when everything (but the removed juice) is in the bucket.
4) Ferment what is in the bucket (stir a few times each day)
5) Starting around day 4 of the ferment, after the vigorous activity has started to subside, add 1/2 gallon/day of the reserved juice to the bucket (once a day, or 1 quart in the morning, 1 quart in the evening when you stir)
The reason for the headspace is for foaming. The highest risk is around day 2. By the time vigorous activity has subsided, then you can slowly add back the reserved juice. If you add the reserved juice too quickly, you risk a volcano.
The reason you want juice only in the reserved amount is that you want maximum fermentation time with the skins and other solids.
If it does not all fit in the 6.5 gallon bucket (but would fit if you removed the solids), then around day 7, remove the solids, and add the remaining juice. Let that ferment to completion.
If you still don't have enough space, then ferment in 2 batches (defrost 1/2 now, and 1/2 later). You can use the lees (sludge) from the first fermentation to start the second fermentation.