Well, I just hate to see a post go unanswered
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I don't have any experience with the "chassis strengthening kits"(boxed trailing arms, etc.), but my '73 had a set of Brad Mayeur (?sp?) stiffeners installed on it (was going to be a V-8 conversion) when I purchased it, talk about 'heavy duty', those babies are close to bullet proof, literally! I believe that there's a web page that describes the entire installation, and by looking at mine, it looks very DIY if you've got a welder. As for the other 'stiffeners', Pelican sells them:
http://www.pelicanparts.com/catalog/shopcart/9144/por_9144_chasis_main.htm
And it's good for everyone to support Wayne and the boys' at Pelican.
Okay, I found the site with the info about the Brad Mayeur kit installation. (at PP of course)
http://www.pelicanparts.com/techarticles/914_longitude_repl/914_longitude_repl.htm