I've been off grid for a few weeks, and I'm just reading about your project now. Go Chris!
I had to build the stab-mounted rear frame from vague pictures, and it's an important piece. Steve Naugler mounted the blower on the back of his E15 without it, but I didn't like the cart grinding against the blower housing on turns. I'm glad Harold was able to find you one.
I'm using 6" (I think!) wire reinforced transparent hoses which makes it theoretically easier to spot clogs (which I haven't had) and lets me monitor the flow of chopped leaves at a glance. The hose runs from the front mower deck along the right side of the tractor and up over the rear fender and has very little ground clearance but doesn't drag. Works GREAT, but would probably be even better with a belly mower.
My big leaf collection trailer is pretty rusty and I'm planning to cut the cap/hood down to fit my slightly smaller cart and scrap the big one. The big one holds vast quantities of chopped leaves, but I'm tired of having two carts cluttering up the stable... I have to drag everything out to work on my casting foundry.
The chopper turbine blade is very heavy, and the better balanced it is, the happier you will be, I think. Vibration is an issue, and noise too. I strongly recommend you spend some extra time on careful balancing (although I haven't any techniques to share, Steve did mine).
--Charlie