Congrats - that's quite a deal. I got a slightly better one though. I paid all of $50 for one of those last year on eBay. It was actually listed twice; didn't sell at $100, so the seller relisted it and lowered the price. Key point - He had no reference to a Valkyrie in his description.
I have the Honda version on one bike and the Yamaha version on the second. They are the same.
You will need to extend the power harness regardless of where you mount it. You can cut and splice in new wires or purchase the appropriate connectors and make an extension harness. I chose the latter after finding the connectors at
http://www.vintageconnections.com/. IMHO it looks much better than splicing.
Sierra-mc.com sells a spliter that allows you to use one antenna for both CB and AM/FM radio. Works great.
The CB will fit under the seat; you'll need to spend a little bit of quality time with a dremel to modify the the plastic box though. I seem to recall cutting some off the right side to provide a place for the harness to exit.
Good luck. It's relatively simple.