Longhorn box question

[Bam Bam 56]

I have a 1970 Longhorn that i am doing a frame off resteration. It was my grandpa's farm truck for over 30 years so the box sides were pretty much shot. When i dismantled the box i was able to save the box extensions. I have been told that i can just use box sides off a regular long box and just modify the front to attach the extensions. I am just looking to find out if this is in fact true. My concern was will the wheel wells and bed mounts line up since the frame is about 8 inches longer on the longhorn. The box sides i would use are from a 72 long box.