Better late than never I suppose.    We already knew this:

Cars: 1993 Porsche 911 Carrera 2 Speedster, 1995 Porsche 911 Carrera RS 3.8, 1986 Porsche 959 Sport,
Rewards: 56 gold, R$56,000
Location: LEGEND
Unlocks from: 24 Trophies in Supercar Elites
Maximum Required PR: 56.5
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Just for the record the rewards split 7 @25%, 11 @50%, 15 @75%, 23 @100%, and you need the 959 for its Showcase and Time Trial plus the last three Cup races.  When available the 959 is unlocked by tier 12 (roughly 54% and PR24). For just 75% you need PR41.
Also, if you hadn't already noticed, the Monkeys have sneaked a couple of 959 events into Supercar Classic Clash, so if you'd completed that there's now a recomplete required (for 3GC reward).

Finally, this how far you can get without the 959 and with the others fully upgraded: