Bloody ell!!
I just looked at post 85! Looks awesome Mark! Congratulations and welcome to the Z Club. I love it, nice to see attention has been spent on the interior, which is so often overlooked on these conversions.
Please, more pics when you can, stick em in your gallery!
There was a kit called the Alpha One that I think was around during that era, I guess it could be one of those. The Colin Shipley book has a section on them, I will take a look later and report back.
The tank would have originally been under the car, not in the boot, so I guess that is a feature to make it look more authentic.
As for converting the rear hatch, it's all up to your personal preference, I think it looks great as it is, but if its a bug bear for you then you can always do it.. I guess it should add value to the car also.
Keep an eye on the chassis rails and waxoil regularly.
I bet you got some mad looks up the M4!