Thanks 69Z28-RS,
No chance of finding a Chevy factory tool down here in Australia unfortunately.
A great bloke (Jim Shea) gave me some advice via email and I've made a tool to remove the cover but it's stuck on so tight it's bent the tool (as you can see in the pic). I'm now trying to find someone to make one out of stiffer steel and try again though I'm pretty sure it'll just unroll the lip.
Here's what Jim Shea sent me for future forum reference......
Method 1
Remove the turn signal lever, hazard warning button, and turn signal switch. Now take two fairly large tools (such as screwdrivers) and insert them through the turn signal lever and hazard button windows in the Housing can. Now try to carefully pry the Housing can up and off the Housing Assy.
Method 2
If the above pry bar method doesn’t work, here is a tool that can be fabricated to remove the cover. The link shows the tool that is designed to slip under the rolled lip of the cover to remove it.
http://s2.photobucket.com/user/JIML82/media/1967-68TiltHousingTool_zps8d7af4fd.jpg.htmlIt uses a steering wheel puller and three circular sheet metal parts that make up the tool. All three sheet metal parts were made from electrical conduit box covers. The circular plate with the nuts attached is smaller in diameter than the inside diameter of the rolled lip of the cover, so it will slip down inside. The two half-moon parts are larger in diameter than the circular plate and large enough to fit under the lip of the cover. But since they are split, you can work each one individually down inside the cover and under the rolled lip.
Place the circular part inside the cover with the nuts on the underside. Now work the two half-moon parts down inside. Align the bolt clearance holes in the half-moon plates with the nuts on the circular plate. The puller should have a nut threaded on each of the two bolts. Thread the puller bolts into the nuts on the circular plate. Tighten the upper nuts down against the half-moon plates and the circular plate. Use the puller to draw the cover off of the housing.