Peter,
I don't suppose you know any former engineers at the UK company Cathodeon, which ended up as a subsidiary of Pye at one point?
I have a nice CW crystal filter in the junkbox. The seller said it is a "NATO pull" with a center frequency of 1750kHz and a bandwidth of 400Hz, and was made by Cathodeon, but he had no further info including crucially the impedance figures.
(I realise that 1750 is an awkward IF because harmonics of the IF would hit the bottom of the HF bands, but there are ways to work around that, including designing the BFO carefully, I am told.)

I am aware that with the right test equipment and circuitry, these impedances can be determined experimentally. But it would be simpler just to find a spec sheet!!
Edited to add: Concerning compliance with emissions rules, an antenna tuner is probably in my near future. That's because although my equipment is able to operate on 80m, I don't have space for a fullsize 80m antenna which is why initially I'm on 40m monoband. The 80m aerial may just end up being an inverted-L, or an end-fed, awkwardly routed sloper of some kind. (There is a separate thread on that in the Antenna forum.)
There are dozens of designs for homebrewed antenna tuners in the literature....
73 de Martin, G3EDM