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.
Gearbox internals are in stock.
Fully tested and fully working low mileage pre-owned gearboxes are also in stock.
Just drop me a PM if I can help further.
Any Nissan garage will service your 370 as well as independent Nissan specialists.
Prices would have to come from your selected garage as they vary considerably.
These guys should be able to help:
http://cougarstore.co.uk/