You know the Stumpy SKyline Box will fit.
Andrew made it fit on your old CA project. You won't need to change the gearbox bellhousing though. Any L Series box would fit. Maybe the L16/Stanza one would be the 2nd best fit?

The L20B is a bit of a pig in a 200B, but with weber/webers, and in a 1200, it would be cool.
Get a lightened flywheel (-10->20%) as flywheel weight is proportional to vehicle weight, for getting up hills.

Bonnet clearance could be an issue, as it can be in a 1600 if not mounted low enough. The L20B is about an inch taller than the L18.

The injected ones are much rarer. I've never seen one, but I know they exist.

I'd get a freshly reconditioned L20B, and buy some nice matched carbies and give it a go. Should be possible to get it going well for under $2000, but $3000 is more realistic, for a fresh engine.
$1000 for engine without intake/exhaust.
Get some Webers and extractors $800.
And $200 for a Tailshaft.

You might be able to get a worked L20B+gearbox with good carbies and extractors for $1200ish, if you snoop around on ozdat classified for long enough.

Will need custom mounts for engine and gearbox.

Will need an engineers certificate.

Chris