A 1975 bike will have a freewheel, not a cassette, and it will have 5 cogs on the freewheel. The rear dropout spacing will be 120 mm, rather than the 126 that was standard in the later 80s or the 130 that is standard now. That means you can't put a modern wheel in it. So if the rim is shot, it's a good chance to learn to build a wheel on the old hub. You can get 27" rims. If you need a new chain, ask for an 8-speed chain and that will work fine. Be warned that putting a new chain on with an old freewheel may lead to skipping, so locate a 5-speed freewheel before you replace the chain.