New exhaust system won't seal

Hello remembers! Here we go: 1999 Peterbilt 379exhd. Just had a completely new 6" Lincoln Chrome exhaust installed. 114" stacks, 90° Rod Pickett style elbows & the chrome Y-pipe. Everything under the hood & truck mounted up great & sealed aganist leaks as expected. The problem is the stack to elbow joint is leaking. The elbow slides into the stack approx 6 inches, the fit is tight, the clamps are tight. Even used Lincoln Chrome's recommended practice of sealing the joint using Permatex RED RVT High Temp Gasket maker/sealant on said joint. NOPE..... still leaks. I'm just a little frustrated & pissed off!! A $2400 exhaust system SHOULDN'T leak like this, but I nor my mechanic know what else to do.... ANY ideas? Anybody else have an experience like this & what was your solution?? Thanks in advance.

 

I appreciate the ideas. UPDATE: The passenger side stack/elbow joint sealed itself this past week (YAY!!!). Drivers side, NOT SO MUCH. Took truck back to mechanic yesterday. Bought a tube of the Permatex High Temp Copper RVT sealant. They actually got the stack & elbow apart this morning, cleaned the parts up like new, applied the sealant, put back together & re-installed on the truck. He is going to allow it to dry the remainder of the day & apply another small bead around the stack to elbow joint & let that set over the weekend. Re-install the clamp Monday morning & we'll see what happens. The stack & elbow are as round as they can be. If they weren't, you couldn't put them together. The stack fully seats on the elbow too. You wouldn't think a fit that tight, with 6 inches of elbow inside the stack, would leak. But it does. IF this doesn't solve the issue, Lincoln Chrome will have to come up with something different!