Texas, I think this reproduction part
1955-80 Seat adjust return spring from Classic Parts should handle the task in your truck. The spring (1st pic below) looks of the correct design and says it has an overall length of 12".
The 2nd pic is an extract from a 1929-57 Chevy Master Parts Book that shows the 1951-55 1st Series spring as 12-1/2" long free length (GM# 3698449 as shown in the FAM you referenced), while the 1955-57 2nd Series (includes later trucks too) spring has a free length of 11-51/64". Just a bit shy of the 12" listed for the repro item, but the 12" should IMHO work for either application. Not a bank account breaker. Don't forget you need one for each side.
