BORGESON steering box

You can buy the splined adapters at Summit and simply weld them onto your existing steering coupler.
I recall having paid some $10 for mine years ago, but I see they've almost doubled in price already.

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I think BAC simply sells you a brand new complete steering coupler unit with this adapter already welded on. Has some markup, but for this part it doesn't seem that much out of line.

Retrofit Power Steering Box From Borgeson - One Good Turn