Hi Bill!! Great effort there finishing the manifold. Many people that have never done this sort of fabrication will not understand the effort that it takes to make a manifold. It is no monster garage show where everything is done within an hour long show. So definitely a good accomplishment to build something like this.
On some welding pointers
Not putting in the appropriate amount of heat to achieve full penetration to avoid inside the tube slag is not the right method to use. The weld should have been purged on the inside for strength. With the right amount of heat applied, you will be able to get full penetration with the filler rod and with the inside purged with argon, the inside weld will not turn into crusty slug. With inside purge, the inside weld will blend over the inside wall by itself without needing to be ground down afterward. I know this is hard to explain. I also know it will cost more money to do an inside purge. My big bottle of argon each cost around $150-180 and when I am purging a manifold you can definitely see the argon gauge going down fast. More cylinders, more purging time as each runners is purged and finished weld separately. On top of that having 2 separate argon regulator setup, having more than just one bottle of argon and buying the rest of the purging setup also add to the overhead.
Off course...
Not getting full penetration will also encourage cracks to form from uneven hot/cold cycle expansion and contraction at the weld seam. Off course you can always weld it later (fix it) but by then you will also have to deal with the carbon contamination from exhaust service. Nothing frustrate me more than dealing with little volcanoes of weld that erupted when you accidentally weld over these contaminants.
I am not trying to be too critical, just offering a different point of view.
