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I didn't, I had a long think about it being slightly concerned that the DT is probably only ~0.7mm there (was thinking half a section of tubing between DT and box section to give additional thickness) but didn't bother as I didn't have any appropriately sized tubing and I'm impatient. I suspect it will be fine with the weight that's typically being carried. But as you say it makes sense to reinforce now anyway, before paint, additional weight isn't a concern. Might do a triangular gusset similar to the HT brace between the box section and DT.
@M_Vatwork sweet! Sounds a good plan. As you say alignment could be awkward. Replacing the HT ended up being very simple and quick in practice, the majority of the removal being done with a grinder.
Looking good!
Did you reinforce the DT where it meets the horizontal box section? Everything else looks super heavy duty, but to me that looks looks like a potential weak spot... it is the thinnest part of a butted tube too
Thought I would point this out now before you paint, as it would be relatively easy to braze more steel on in its current state.