Hello, working on a 1991 F-250 XLT Lariat with a 351w motor. I need to remove the secondary air injection tube on the drivers side in order to access valve cover for gasket replacement. Secondary air injection tube is rusted tight where the four lines run into the exhaust manifold. Any tips on getting these out? I can get either the 5/8 wrench on the smaller part or a 3/4 on the larger, but they're not moving and Im just going to strip them with brute force. They're currently soaking with PB Blaster.

Secondary air injection tube

parts I need to disconnect

 

If you have the notion that you want to be able to reuse those parts, I would recommend penetrating oil, heat, and a lot of patience.

If it was mine, I would ditch the AIR injection and the cast iron manifolds and run some shorty headers.

 

gas axe with a very small tip, get the outer part of the fastener cherry red, usually they will come apart.

 

is quenching a valid idea or is this scenario one where it would crack something

 

I don't quench, but I honestly have no idea.