school me on a 5.9 magnum motor.

My TTI step headers have Magnum port shape. I was told at the time that only way they come. So the header flange hangs into each of the 4 LA corners ports, the two front and two rear ports. But its lower corner so impact to flow not as bad. I had a welder add a bead to header flange to increase flange to gasket surface so easier for gasket to seal.

Point is LA and Magnum exhaust ports are different but close. You might have to use some high end squishy header gaskets to get 340 manifolds to seal. Or maybe add JB weld around outside edge of front and rear ports to provide a gasket surface. I believe the 4 middle ports match its the 4 outside on lower corner on each of those 4 thats needs looking into.

Magnum heads are prone to cracking so look out for that.

Magnum heads flow great and factory roller cam decent compression. 360 Magnum very nice starting point just freshen up cam with matching valve springs make some good power.