Exhaust/muffler connection

[30coupeguy]

When I removed the engine, the exhaust manifold connection to the muffler had a generous amount of hard substance around the mating surfaces and inside the clamp.

I looked kinda like furnace cement or something similar. Apparently it worked to seal the connection. I put it back together without any thing other than the clamp and I believe there is a leak.

Should a high temp sealer compound be used, and is there a type that works best? Is there a better idea?

thanks

burt