Instead, I highly recommend you try one of the newer-generation leather boots that are suitable for climbs like Rainier. A good example: Salomon's Super Mountain Expert ($250; www.salomonsports.com). These combine the light weight of a leather boot with the ice and crampon performance of a plastic. I wore a pair on Rainier in July and thought they were the perfect summer boot on all but the most technical sections of the mountain. Very comfortable and warm, but much lighter than plastics. Crampons worked great on them, too. And break-in was minimalI basically took them out of the box, laced them up, and climbed the mountain.
Another boot to look at is Tecnica's T-Rock ($320; www.tecnicausa.com), which employs carbon-fiber and ballistic nylon materials to create a boot that is light and flexible, but also plenty rigid for crampon use. The T-Rock is designed for climbs like Rainier.
In any big-mountain boot you're going to struggle with fit. You might want to order a couple of different boots and just be prepared to ship one or all back. Try them with the right sockslight Coolmax or similar wicking socks, and a heavyweight wool sock such as the SmartWool Expedition ($17; www.rei.com). If needs be, use an aftermarket insole to soak up some extra space. The folks at Superfeet make a "high profile" model that is meant for folks like you ($28; www.superfeet.com).
Good luck! And I hope you make it all the way to the top next year.