With the Magnum, the only consideration you may want to keep in mind is it doesn't "drive" like alot of other creekboats on the market right now. It's almost more like a very high-volume playboat-- and what I mean with that isn't that it performs
like a playboat, but that it really is a planing hull boat at heart: so you get the carving, the hard edges, the maneuverability. My problem with it wasn't so much a true "problem" as it was a matter of personal preference-- I'm just more of a displacement (or semi-displacement) kinda gal when it comes to creeking (that may be a product of location, though: in the SE, a planing hull finds every rock in the creek).
The Magnum is a very
fast boat, accelerates without effort, punches holes like a dream. But for me, it's a *big
* boat, and at my weight, I found it didn't hold a line as well as alot of other creekers (I have friends paddling this who are about the same height/ weight as the original poster, & they haven't run into that issue). All that being said, it's a fun boat to paddle, and at your size, it's definitely worth consideration. I think it's a great boat for a larger person (read: normal-to-larger sized guy), and would probably be well-suited for western river conditions. Danger is right, though: Give it try!!
(BTW, the only way to avoid making a "mistake" with a boat is by demoing around and determining what you like, don't like, or are really looking for in a creekboat. My advice to you would be, when the waters start coming up for you guys, you get out there in the handful of boats that interest you, and work from there. So what if ya' spend a month or two trading out boats with friends or demoing??
All of that just brings you one step closer to finding the features you're looking for, the boat that fits you best, and in the end, the boat you're most comfortable in on the water) Good luck with it!