The tamiya axle alone is def not wide enough. You could make some extenders to go on it at the hexes but your suspension mounts and all would be no where close. It would take quite a bit of modification to either extend the case or making wideners and al new mounts.
K&S makes aluminum and brass tubing that you can press bearings into the ends of and just use rod as you are now. Only they will ride on the bearings instead of the tube itself. It's available at most hobby shops or online, they also do cut to length so you can order direct if needed. Here's a link to the sizing
http://www.ksmetals.com/resources/cuttolength.pdf
Or check out
www.onlinemetals.com for all different sizes shapes etc you'll find tubing there as well. This is where I but most of my metals for my hobby stuff if I can't get it through my main supplier at my shop. Pricing is good too...
--Dan