The basement shows no sign of being finished, and so, I am continuing to refine the plans for 622. One of the challenges will be maintaining the relationship between the two frame halves while the insulating epoxy layer sets.
I had been thinking I would add sacrificial tabs at the ends of the frame, and hold them the correct distance apart with a spacer. The dimension of that spacer would be critical, and alignment pins through the tabs could help with the horizontal alignment.
Then I thought of the idea of a sacrificial jig for the whole frame. The two frame halves would fold up from a common bottom pan that should hold them in alignment both vertically and horizontally.
The more I play with this idea, the more useful it becomes. For example, by making the pan the right depth, I can locate the firebox vertically. And as I write, I realize I can even screw it to the pan, so no clamping should be necessary!
Now, if only I could think of a way to ensure the bearings were really the right distance apart…
Pembroke:87 
What are your thoughts on horn block guides being soldered into place using jig axles? A solution to the alignment question you pose?
That’s the approach I took with #10, and the Brits swear by it. Frankly, I found it a fiddle, and I’m not sure it’s the best approach. Having said that, I will probably do it again anyway!