The interstage would need to be 50k on the primary or thereabouts but these are not really obtainable or affordable.
an A-18 may sound more correct than a modern replacement, even with the shift in operation.
I worry these two statements may cause undue confusion. The Sowter BA6A interstage is 1:1, is readily obtainable, and performs very well as shown in another thread where it was tested.
As to affordable who knows. Anything vintage and UTC is now crazy high. How long will it take you to build your "bargain" BA-6A? That's a lot of sweat. Based on what I saw during testing the 1:1 looked best overall for the design. If you already have an A-18 then why not. Or you could easily sell it and buy the Sowter or try the Lundahl. Besides, most people want to lower the threshold on the big tube limiters and you're already contending with a 10W output. When you consider all the hours of work you'll put into it . . . . .
You can spend 200 on the Sowter interstage and still build a BA6A for probably a lot less than what most typical vintage ones sell for. That's another way of saying question yourself if the need to constantly swerve from the perceived big wallet bite is running you ragged.