Minor update here - I've isolated the noises from the engine. Looks like the bigend is definitely on the way out on the rear passenger side cylinder of the engine, I suspect a conrod bearing. Not much I can do about that right this minute but I will order a set of bearings, unbutton the sump and see if I can get at it while the engine is still in the car for a quick, dirty, ghetto-style fix.
I found that out by disconnecting the supercharger. This let me run the engine without it turning, and isolated the noise to the bigend. It also had the benefit of letting me diagnose that the supercharger needs a new coupler and maybe also some new housing bearings. They're cheap parts so I ordered some from Mace Engineering in Melbourne:
While I was on their site I had a bit of a look around and saw a whole lot of goodies for the L67... including a water-to-air intercooler plate complete with composite insulating spacer.
BAM! This will also be a quick and dirty install, I'm planning on using some very basic hosing running around the side of the engine bay to a repurposed airconditioning condenser or two that I'll install in front of the radiator. The pump is only going to be very basic - a $17 eBay unit that flows about 15L/min - and will be fitted on the hosing to the side of the engine bay. From all reports this is good for a pretty large jump in power as the poor little Eaton M90 blowers have discharge temps of about 200 degrees celcius.
Lastly, I finally got two new tyres on the back wheels. All I had to sort for rego was the indicator lights which looked like they'd shorted due to corrosion on the fusebox, so hopefully I can get her regoed in the morning.
Wish me luck.