Pre 1989 CR500 engines have a longer intake - which causes the carb to sit back further and touch the shock on the bike. There are a couple of ways to handle this.
You can shorten the inlet boot so the carb sits closer to the engine. This is just a matter of trimming the boot. Before cutting it, verify that it will move the carb enough to clear the shock.
The other method is to change the angle of the inlet so that the carb moves away from the shock. This method only works if your reed valve cover on the inlet is metal. If you are looking for a metal version, look for a Boyesen Rad valve.
Check out the photos below to see how Ross M handled it on his 2005 CRF250R conversion.