Trailing Arm sizes?

[moodybo]

I was at a salvage today and the told the owner that i was shortening my long bed into a short bed! He says that i have to use trailing arms from a short bed because he already tried to sell a set from a long bed and he said they were too long for the short bed to center the wheel!

Does this sound right? Every where i looked for aftermarket ones says they fit all long and short beds! I tried to convince him but maybe he was right? Help me dispute? By the way i was looking for a door not the trailing arms!