I sort of have an update. Not that I played GT7 again today, but I think I've narrowed down which cars I'll be using for the Alsace One Hour and the Kyoto Driving Park One Hour. For the former, I have a Ferrari 308 that I've already modified, so I'm going to put on some Comfort Hards, then tinker with its power/weight to see if I can get it just under 450 PP. As for the Kyoto Driving Park One Hour, I'm thinking of taking a spare Ferrari 458 Italia that I won from the menus - it's Giallo Modena - then solely equipping Sports Hards, and also buying a ballast and power limiter. As stock, the 458 comes in at over 610PP, though I can't remember which tires it comes equipped with. But with some extra weight and limits on power, I could see it potentially working out.
However, regarding the Ferrari 308 I plan to use for the Alsace One Hour, it seems that even as-stock, it's already above the 450 PP limit. But I do have other candidates, including the Ford Mustang Mach 1 '71, as I do have a spare. Then there's a Nissan Skyline GT-R (KPGC10) that I've already customized - including the installation of a widebody - but I haven't done much tuning to it, so that also might work.
Then, for the Kyoto Driving Park One Hour, I could use the Nissan Skyline GT-R (R34) that I won from the menus. I may not have a spare of this one, but I can easily get another at Brand Central. Besides, I want the R34 that I leave as-stock in Millennium Jade, and the one I have is in Bayside Blue. I also have a Mitsubishi Lancer Evo X that I also won from the menus, but it was white instead of red, so I've already installed a widebody on the white one, as I plan to paint it Dandelion Yellow. Otherwise, like the KPGC10, I've only customized it, without doing much tuning, so it could still work. Yet another candidate could be the Ford Focus ST '15 or Ford Focus RS '18, as both come quite a bit below 550 PP, though the ST could very well end up getting more parts installed; it comes in at a mere 490 PP as stock.
So let's recap my candidates:
Alsace One Hour
Ferrari 308 GTB
Ford Mustang Mach 1 '71
Nissan Skyline GT-R (KPGC10)
Kyoto Driving Park One Hour
Ferrari 458 Italia '09
Ford Focus ST '15
Ford Focus RS '18
Mitsubishi Lancer Evolution X Final Edition
Nissan Skyline GT-R (R34)
Definitely seems like a three-way fight between Ford, Ferrari, and Nissan to win my interest.