Hi Stew,
Having built several boilers and made some mistakes I can offer this:
The secret to a successful boiler is cleanliness and having enough heat. The seivert 2943 should give you enough therms.
Do not clean your flanged plates with emery cloth.
Mix plenty of flux with clean water.
Use citric acid, Wilkinsons sell it but you need to ask for it.
If you can get hold of an old storage heater they have lots of refractory blocks to make a hearth with. Less gas that way

Add bushes for the firebox door hinges.
Do not skimp on the chamfering when making the foundation ring.
Use 1/8 rivets for side stays and fit them before fitting the firebox door plate as it's easier to braze them that way.
If you can get hold ALEC FARMERS book on boiler making it's worth the read.
If you know all this already please ignore, the work looks good so far.
kind regards
Steve