I've picked up some info from another site that actually included some posts from a guy who worked in Iola, KS at the IMP plant there. I drive through Iola on my way to LOTO, so I was quite surprised when I learned the history of IMP.
This post on boatdesign.net gives some info, but you have to dig because most of the people on this forum have older, smaller IMP's.
This site was set up specifically for IMP owners, but again, seems to cater to the older, smaller models.
Maybe the 310/330 owners should jump on there as well??