I am doing the same thing between trying to plant and watching it rain.
Hired a contractor to cut in the end wall. To hold the building together after floor removal he put cables at each rafter at the height of the door. I took out the floor myself after the cables were installed. My barn is 30x70, the new door is 24x16. I am also only useing it for storage and will not be finishing the inside.
Included some pics of the project so far, excuse the mess, as I said it's a work in progress.

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DSCF1862-2.JPG (94KB - 918 downloads)

DSCF1855-2.JPG (89KB - 964 downloads)

DSCF1856-2.JPG (98KB - 903 downloads)

DSCF1857-2.JPG (83KB - 938 downloads)