Andy - I don't have a solution as I can't get the 8 tubes to Union into one piece.

I would instead use Rail Revolve to make a tapered, hollow cylinder with a single inlet hole on top. Then attach the 8 outlet tubes to the cylinder, rather than attach to each other. Boolean Union all and add fillets.

Also, if it's possible to arrange the 8 tubes in a circle then you can build just one tube and use Circular Array to get the rest.

Ed Ferguson