You can drop and roll the chaincase. I have a set of templates I believe for it if you would like them. They move the top shaft in to the tunnel. Another option that we did was to put one of the domestic twin chaincases... like from 1997-2001 on them. You can leave the top shaft in place and gain some clearance at the bottom.
The rear skid has a bolt to adjust spring tension which was replaced by tension blocks later on. Same principle, tighter gives you more preload and less sag in the rear.
Triple pipes worked great on the 650, something like 20hp gain or the likes. SLP made a set that worked well, PSIs were great, but thin and blew through pretty quick. I think the Decker triples were nearly an exact copy of the SLP ones.
No one will have them new, but you might find them used. The SLP triples for the 650 were different than for the 600. The 650 set had a 'torque tube' that extended down right by the header flanges. If you find used ones, make sure they are for the 650 not the 600.
sled_guy