You are brave to try this shortbed conversion.
The fun part comes after it's all welded together and you start the body work.
Most guys fail miserably trying this so don't be discouraged if it doesn't work out because the good news is you can buy repros and the 1967 short bed bedsides from classic industries are excellent quality featuring stamped OEM gauge steel.
I purchased them for my own truck when I decided to convert mine from a step to a fleet and they are very nice.
http://www.classicindustries.com/sho...ed-sides-full/