Stew Wilkins does a long range capacity tank for a Z, as used by many in long range endurance ralles.
I ran a 1966 Chevrolet Belair in many Variety bash evens, i had it fitted with an additional 100 litre tank, in the boot, behind back seat on the bit of floor above the diff.
It retained the stock 80 litre tank under floor at rear, but, i fitted a alloy sheet folded tank bashplate, with 1/2" thick closed cell foam between tank and bash plate, to stop stones getting in there and puncturing tank.
A two way switch on the dash controlled a simple two way solnoud valve, mounted inside boot, we'd fill both, use up all the under ank, car would go "cough cough" then switch to auxilliary tank in boot.
Reason for using under tank first, was, n case we punctured the bottom tank in the rough stuff.
Saw a few others put neat holes nto tanks in Gibb River Road!
In the Z, maybe consider keeping stock tank, fit a bash plate, and then, in spare wheel well a circular tank made to be flush with flor, so a piece f plate can be sealed/fitted to floorpan and have it all flsh.
Spare tyres mount,, a couple of bolted in bits of steel rhs with a bloody decent bolt poking up, to maybe enable two tyres one on top f ther, with top one on uside down so the bolt it not too long, to make it easy to get out.
The Peking Paris cars run tyres on a roof rack, gets a bit cumbersome and looks a bit ugly, i reckon.
Oh and use steel rims, if they get bent, you can bash the lips back out with a hammer.
Your pics are on the money!
I really should write a book!