Converting a 8' box to a 8'6" Longhorn box

[Longhorn 69]

The box on my 69 Longhorn has seen it's day as my dads farm truck and 25 years sitting in storage. I have to completely rebuild it and I am trying to decide if I should go with new box sides, or use the standard 8' donor box I have, and repair the few dents and replace the steel floor, front panel, and all the bed cross members, for a wood floor. Also trying to decide the best way to cut and add the front bed side extensions from the old Longhorn box to the new 8' bed sides. Has anyone used the whole front, seams forward, or just cut out the seams and cut into the new 8' bed sides? Looking for some thoughts on this. Thanks