Jake,
I only just caught sight of this thread.
For your info, take a look at
http://forum.fefcholden.club/index.php?topic=25411.0 and
http://forum.fefcholden.club/index.php?topic=25680.0It sounds to me like the rear main bearing oil seal has spun in the groove. But eliminate all other possible oil leaks first.
The only feasible way of tackling a leaking rear main seal is to take the engine and gearbox out of the car. Then you'll have to take the gearbox, clutch and flywheel off, and mount the engine in a stand. Next, remove the distributor and fuel pump, rocker assemblies, pushrods and cam followers. Remove harmonic balancer (you'll need a puller), and timing cover. You may have to remove the Woodruff key from the crankshaft to take off the oil slinger in front of the crank gear. The camshaft will need to be removed, but the only socket which will go through the holes in timing gear is a 1/4" drive 7/16 AF socket. Now you can remove the front engine mount plate.
Tip the motor upside down, remove the sump. Remove the oil pump and delivery pipe. Finally, remove the main bearing caps and the big-end caps (mark them 1 to 6!), and you can lift the crank out.
When it comes time to replace the crankshaft, cut twelve ~100 mm lengths of 8 mm clear PVC tubing and push them down over over the big-end bolts so you don't ding the crankpins. Probably a good idea also to push the pistons and conrods up into the cylinders as far as they will go, lay the crank into place, and then pull each piston and conrod down to meet their crankpins as you go.
As the others have said, replacing a rear main oil seal is a big undertaking. It'd be best to find someone who's done the job before and can guide you safely around all the traps (and there are many of those).
Best of luck,
Rob