Here are a few clarfications:
BH-6 has the same specs as a BCP-15 gapped for 50mA (i.e. 40 henries), but with superior winding techniques. The winding technique - not the core material - provides some treble enhancement.
Neither is offered with a nickel core, since that would reduce the current capability with little other effect. Core material is much less important in plate chokes because they have many more turns of wire.
The solder terminals are exposed, making it dangerous since the choke terminals are at the full power supply high voltage. If mounted topside, the terminals will be on the top - a box "open at the top" will be poor protection. I cannot recommend it, but a much less unsafe arrangement would place the output transformer on top. Its primary is at the 2A3 cathode voltage (60 volts), still above the nominal 24vDC safety threshold but way better than 380vDC (!!!) You would still want to insulate and protect the two primary terminals, since accidentally shorting them to the chassis will remove the bias from the 2A3, rapidly damaging the tube. The secondary, including the impedance switch board if you are using it, is safely grounded when wired correctly.
Both chokes have the same mounting dimension as the stock choke and output transformer.
