Ben;
Welcome to this forum, and congrats on you acquisition!
The '68 1800S (the leading P got dropped and the trailing S was added in '66, when production was moved to Sweden)...you could clean up rattiness of the harness by taping with electrical tape...thankfully, in '68, the crumbling, biodegradable wiring of the later 240s did not yet exist, so you can just clean up terminations and terminals as you go through the car... I recommend using Anti-Corrosive Zinc Paste on all connections after contact cleaning/restoration. See:
https://www.sw-em.com/anti_corrosive_paste.htm
Unless you have previous experience with vintage Volvos, I also recommend getting (at least) a Haynes Manual...it will give you a head start on much service and maintenance work...and asking on the fora is also quite helpful as there is literally a world of experience to draw on, to save you expense, and the disappointment of doing things "the hard way"!
I invite you to visit my sw-em.com site where much is documented.
Cheers from Connecticut!