I'm not sure if a "shoulder bolt" AKA "stripper bolt" would work for you or not. The threaded portion is smaller than the ground portion of the body, so it may concentrate stress if it isn't designed correctly. Unbrako and Holo-Krome both make quality fasteners.
For experimentation sake, you might see if you can use a stud made from "stressproof" steel or 4140 chrome-moly alloy. ENCO has some Gibraltar cut-thread studs on page 764, and maybe the outside diameter would fit your bearings. You could at least try something like that and do a lot of jumping up and down on it
before daring to do any road testing. Cantilevering a decent load a couple inches out on a 5/16" bolt may be pushing it. Maybe you could put one of your trucks on a handtruck-like arrangement and tow it behind a vehicle with a 300 pound sandbag on it. See how it holds up.
http://www.use-enco.com/CGI/INPDFF?PMPAGE=764