Welcome to the site.
Most people have a lot of time and effort involved in developing a suspension kit and it's doubtful that they'll be willing to sell any plans.
If you have the desire to build a kit of your own, take a long hard look at the way the suspension on your ride works and then work from there. Front end's actually pretty easy to widen. The rear end is the one that can be a bit more intensive in design and fabrication.
I'm not familiar w/350's one bit, so I can't give you any advice on them. Sorry.
