I think you will be really really surprised how bad those manifolds leak. Making a air tight weld with a MIG or stick welder is no easy task. I am a pretty good MIG welder, and i HATE welding bead-locks because of all the testing/fixing to fix all the super small holes!! Keep in mind most holes you cannot even see, I have to use penetrating oil to find them.
Also you say that you got good penetration, but it doesn't look like it was prepped properly, which means you didn't get good penetration. With how much that thing is going to shake, I don't think its going to hold up very long.
I am by no means a pro welder, but I was just an apprentice for a few years.
Not trying to be a ****, just dropping off my $.02
PZ