Chop, chop, chop
Do the trunk chop before you plant it out. If you transplant without a trunk chop, you've wasted a growing season on the new apex.
I would, however, not do it now with a beech. I would hard prune the top back to one leaf per branch now. Leave it in ground until early next spring. Dig it up then, trunk chop it and plant it out. This approach is more conservative given beech can be reluctant to back bud. That said, if the tree is vigorous, you can be bold and do all of this now.I wouldn't wait much longer. June marks the end of trees' most active growing season in the Northern hemisphere. After June, trees for the most part, begin preparing for the next winter--they set buds for next year beginnning in June. Shorter day lengths signal this shift...