I'm in middle of finding parts for this exact mod

When i get back from work i'll be sourcing some EF-EL series thermos and start fitting them up
What's this serpentine conversion you are referring to?

Single belt configuration instead of 3, like on most later model falcons.

Not sure of the gains, but you need the belt tensioner EF EL XG XH, put in spacers for the air con compressor, different alternator brackets, different power steering or change the pulley.

I think that's it

depends on what manifold he uses, if he keeps the log manifold he will need the XH belt tensioner as it was designed for use with the log.

You will need all other accessories off the front of a EF or EL water pump, alternator, power steer pump etc. You will also need an EF or EL radiator as the bottom rad hose is not the same diameter.

This setup creates less drag on the engine and you should notice a small gain. But not as much gain as you will notice by taking off the clutch fan.