Triumph Dual Carburetor Adapters were the way to go back in the day. Converting single carburetor heads into splayed dual carb intake as Bonneville T120.
They fit pre unit 500 and 650 cc heads and unit 650 Tr6 and 750 single carb heads.
Originally made by Webco in Los_Angeles. I am glad to say, they are still made here in LA
Maund Triumph dual carb adapters are not distributed to any vendor. You will find the casting quality and fit, superior to lesser imports available. Available at maundspeed.com