I'm in to watch the build. I raced an HRC 628 in the California desert for many years. Great old girl; one of my all time favorite bikes. The key to her successes? Keep her cool and she will run like the wind. Also watch out for the subframe. It will crack with the extra weight of a rear tank. Make sure you add some gussets to strengthen the sub mounts to the main frame. I modified mine to be able to unbolt the subframe for easier access to servicing the shock.
Good luck on your project. -Jerry
2014 KTM 690 Enduro R, 2014 EXC500, 2012 BMW GTL1600, 2015 BMW GS800 Adventure and 16 other motorcycle projects in various stages of completion. It's an addiction thing.