One must remember, the 1968
have a "4 inch" frame. Unless you plan to build another frame under the current frame you will in a matter of a few miles on a bumpy road start to pop rivets and make the rear sag issue of same type trailers look good. Of coarse, new axles are a must but will make little difference.
If you want to build something like this, get a newer trailer with a six inch or large frame to carry the load.
Just my thought, your results could very.