This pipe threading simply doesn't give!

[71-440]

Liquid nitrogen on the pipe fitting.

[Mopar5]

They make pipe nipple extractors

[Brads70]

crack open a fresh can of "open sez me" 

[jimynick]

Bolt the manifold down, Weld a nut on the end of the fitting. Hit the wrench with a hammer to jar it (yes right after welding the nut on to utilize the heat). Aluminum and iron don't play well together.

That's pretty good advice. I'd also use an impact gun on the nut, so make sure your welding is up to snuff. If it moves whatsoever, don't stop, as quite often, you'll bind it even tighter if you do. This is basically blacksmith work and their methods are time tried. Use a good teflon tape AND liquid thread sealant when she goes back together- and don't forget to shove something non-collapsible inside it as advised. Good luck and heat is your friend, just remember it's aluminum, not iron. 

[jbaha]

Get a torch and weld a nut! Aluminium doesnt contract when heated hahaha, any "normal" material expand just like boiling water (you know molecule re arrangement etc...) . Plus, alu has a melting point waayyyy lower than iron, should be easy to heat up!

Just don't pour water on it when you'll be done with it to cool it down, please... 

[kawahonda]

I finally got the fitting off!

Teflon tape or anti-seize?

I'm also wondering where to source the heater hose fitting as the one from Moparts isn't right for the LD340.

It appears that I need a 1/2" hose end, and something larger than a 1/2" NPT fitting. I'm guessing it's 5/8".

The one I got from Moparts is 1/2 hose end and 1/2" fitting and it just slides in the hole.

[Chryco Psycho]

Roseville should have the fitting , 5/8 is uncommon in a hardware store