No STC is necessary. Item 404 (d) on the Type Certificate is:
Adjustable, hinged front seat, Champion drawing 7-1261. 7ECA, 7GCAA, 7GCBC
Item 404
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Installation drawing Champion 7-1499 Adjustable seat installation. 7ECA, 7GCAA, 7GCBC, 7KCAB
I suspect that ACA has drawing
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available for sale, as well as a line on the seat itself. Also look at the Fly Market on this site.
Install the seat, make note of the W&B change (if any), enter in logbook, and fly away. You may need an A&P to sign off if the W&B changes. I forgot to check on the TC for any change.
The full type certificate is on the FAA website at:
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/
Jon B.