Buxus are very hardy.
Most boxwood will certainly cope with top reduction. The new growth from old trunks shown above is typical of a recovering buxus transplant, as is the amount we can chop the tops and expect the tree to survive. leaving some green foliage is an advantage to help the tree re-establish but boxwood usually seem to have enough energy stored to do it, even without leaves.
They also cope with massive root reduction which means you don't have to retain massive roots when transplanting. Many of the Buxus we have transplanted just had a few stumps of roots and still managed to survive. They'll even survive splitting the clump into smaller units as mentioned by
@Deep Sea Diver so if there's far too many trunks from ground level, look at the possibility of splitting.
The biggest challenge in trees like this is working out how to style the trunks after they have re-established, but that's a year or so into your future. You can remove obviously redundant and useless trunks and branches at transplant, but leave final style decisions for after it's happy and healthy again.
Don't underestimate the time and effort required to get these out of the ground. They do tend to have extensive roots that are all very hard so you'll likely need an ax or sawsall along with your shovel. Cutting side roots is usually the easiest part. Cutting the roots growing under the trunk takes more effort.
@leatherback mentions that trees this size are surprisingly heavy so please work safe and allow for that weight when getting the tree out of the hole. The less soil on the roots the better - both for handling and after it goes into the container (garden soil can retain way too much water and cause root problems down the track)
Also need to be realistic about what sized container you'll need. Even after cutting roots back hard you'll need a surprisingly large container to hold the remaining root ball - and a matching quantity of potting mix.
Is it worth the time and effort?
I'd say yes. Even if you don't manage to make something from either of your trees, there will be others who would love to get hold of advanced trunks and take on such a challenge so you should have no problem moving them on, once re-established in pots.
