Rally is hard on a bike. You will crash (a lot) and scratch and damage the bike. If you love your 640, think very seriously about using that. If I was you I'd consider buying one of the 525s on the market that's fully configured, and you can always sell it afterwards. It's much cheaper than doing it yourself - unless that's part of the fun.
I'm 6'3" and the KTM plastics can be made to fit fine - handlebars in frontmost position and use Fastway pegs in the drop position if necessary. I didn't even use drop pegs on my bike and it was perfect. The 450 was actually the best rough touring bike I've had too!