With stock shocks there are no problems using a modern o-ring chain on a 350/400 four but full length spring aftermarket shocks don't have much clearance and the thicker chain (thicker than plain chain) may hit the shock. You don't need a super strong chain like litre sportbikes do, a midrange name brand would be more than strong enough. Normal non sealed chains are certainly ok as well - if there are globs of lube hanging from the guard you probably take decent care of the chain lubing.
Sprockets should be replaced along with a chain, sprocket wear is not entirely obvious as the chain and sprocket wear together. The basic test is to pull on a new chain at the back of the rear sprocket, it should not lift up much at all. But worn sprockets will damage a new chain, as will a worn chain damage new sprockets. After decades and many miles, assume they are well worn.
JT sprockets are good in my opinion. OEM are no longer available but can be found NOS at rather crazy prices. Aftermarket rear sprockets will not have the cover plate but it isn't really needed.
Avoid cheapo chains: these can be industrial chain meant for things like conveyor drives and they stretch rapidly with motorcyle stress. These are cut off a long roll and sold either bare or in a white box. There are also counterfeit name brand boxed chains. Buy from a reputable dealer.. ebay, amazon, alibaba etc are full of scams.
Expect to pay around $50.00 for an adequate chain. You could pay well over $100 but you don't need that strength for your under 40hp 400.
