What is a Modern Classic Car is a very subjective and debatable subject with no definitive answer.
In answer to your original question, the 350Z is not termed as a classic for insurance purposes.
This is a good read: https://drivetribe.com/p/what-defines-a-modern-classic-XHDlX2PJQH6KRLDhyn9nBQ?iid=ExJzKzv0S5yqTWL3HoT12A
The definition of classic car varies from person to person and country to country throughout the world.
In the UK my view has always been that any car over 25 years old, may be called a classic.
Talking from experience:
Leave the sub frame alone.
Swop out the gearbox and diff for solid bushings.
Avoid Whiteline as they are made of chocolate.
That is all that you will require for your set up IMHO.
Yeah, they will work.
So might these, but not OEM. https://www.ebay.co.uk/itm/Pair-Rocker-Covers-Gaskets-For-Nissan-350Z-VQ35DE-Infiniti-G35-3-5L-03-07/332573817190?epid=3008443662&hash=item4d6ef1b966:g:YQ4AAOSwGstammB3
Pity that they are not original, will cost circa £500.00 to get them diamond cut.
I always think that painting or powder coating these wheels is sacrilege.