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
KTM- 2014 500 EXC, 2009 690 Enduro R E BMW- 2012 GTL 1600-sweet, 2011 R1200GS Adventure Honda- 2000 XR650R- 2006 XR650L- Yamaha- 1972 XS750 Tracker, 1971 RC5- BSA- 1945 M20, 1969 BSA 441 Victor . The list goes on but why bore you... 16 and counting. Its an attachment issue.