There are several ways to determine if the chains you ordered are adequate. Here are the two easiest.
1) Use the specifications provided by Curt. They advertise the chains for up to
1500 lb tongue weight. Using standard industry practice, they pair to
1500 lb WD tension bars. Your existing bars are nominally listed as 1400 lb bars. Thus by this method, they are a good match.
2) Most chains use standard industry markings to identify the chain grade. Standard grades are 30, 43, 70, 80, and 100 markings are either 8 or 80 to signify Grade 80 chain, 4 or 43 for Grade 43 and so on. This and the chain diameter can be used along with a table from the internet for example this one
https://fulcrumlifting.com/chain-grades/ will give you the working load limit. From there use the general rule of thumb that a particular pair of tension bars are designed to safely generate sufficient torque to redistribute up to 50% of the nominal tongue weight. In practice most will actually be set to redistribute between 20 and 35% of tongue. Now the general formula is that each chain will be tensioned to between 1 and 2.5 times the nominal bar rating when in use though 2 is the more likely number. For yours that would be 3500 lbs max working load and the 5/16 chains should be grade 43 or better. Note that the actual breaking point is at least 3 times the working limit when the chains are new and defect free.
Let me know if you have any questions or if I made any obvious errors.
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