Terry, I'm just looking at this post for the first time, so I'm a bit late to the party.
On my Light Pewter Mach 1, I chose to use Eastwood 2K under hood black epoxy. It has a texture to it, but I have found it stands up very well and not too hard to clean either. Downside, it's expensive and you'll need 2 cans at least for the engine bay. I have no idea if Eastwood paints are available in Oz, but I like their products. For the underside, a former Mustang restorer I know, always used "Lizard Skin". Again, no idea if you can get that there.
In reality, the underside was painted "Slop Grey", which could be any shade of grey to black which is what you get when all extra paint was mixed together. Mine is untouched from the fire wall back, which is what I chose to do, so it's not possible to see what color it actually is, but when I changed the fuel tank, the underside of the trunk floor is a bluey-grey shade. So your choice.
As for Light Pewter, Code V for 71 and 5A for 72, PPG # 2287 the formula was not available in Canada, so mine was "computer camera" matched from un-faded original paint. It's a tricky paint to get right and hard to match IF you ever need to touch up a scratch.. ask me how I know! Perhaps you're luckier in Australia and "regulations" won't screw you up.
These are just my thoughts on the subject.