Not shown on the drawings?

I don't know the particular loco but the fastenings are often screws (small hex-headed) or round-headed rivets, spotted from the chimney base flange, and the tapping or clearance holes as appropriate drilled radially to the smokebox axis.

An alternative, shown in Martin Evans' book, is a two-part chimney, the visible part being a sleeve over an inner liner/petticoat-pipe with a flange inside the smokebox, secured by small countersunk screws. The outer chimney, apparently a push-fit, covers them.

Some designs might use the petticoat-pipe as a sort of "nut" though I have not personally seen examples of that.

Have a close look at what other builders have done, preferably on similar classes. Even better, what did the full-size B1s have?

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The key point is not only is the finished chimney being properly radial and vertical to the smokebox, but the blast- nozzle being concentric with it. I have made a simple gauge consisting of thick, turned discs grub-screwed to a stout steel rod, for assembling my steam-wagon's equivalents.

My 5" SPEEDY has a flared blast pipe silver soldered to the smokebox and a cast bronze chimney slipped over the blast pipe and riveted to the smokebox (stops onlookers pinching it !).