I have done it without removing the manifold but only because I was several hundred miles from home on a "service call" to rescue my son's car. I vote for removing the manifold.

I would suggest removing as much crud as possible by striking all over the outside of the case to knock as much stuff loose as possible and shake it out. Run a 3/8" drill bit in the top hole (by hand) to ream it clean.

Then plug the top hole and fill it with gasoline. Tilt it several times to defeat an air lock that might prevent a complete fill up. Let it soak over night before pouring some of the gas out. Cover the bottom hole and agitate it to loosen the accumulated sludge before pouring out the rest of the gasoline out. Reinstall with a new Oring

Randy