I'd like to thank everyone for the help and close out this topic with an update. I decided the third time would be the charm, and took another whack at it.
Cleaned the part (again!), plenty of flux, some more solder - and from the outside the joint is completely filled. I can only assume/hope the inside is well filled as both the supply and exhaust passages cross that solder line not far from each other.
I think doubling the amount of flux used in the past probably helped a lot?
Now onto machining this thing to final dimensions when I can find the time.