To me it sounds as if he just wants to hook up the Lamda supply to the radio to supply the 250 vdc to the radio.
In that case all that is necessary is to hook the ground and B- connections of the Lamda to the ground connection on the existing supply, which it sounds as if he's still using for the filament power, and wire the Lamda B+ directly into the B+ input to the radio. The B+ from the existing supply won't be being used at all. Connecting the Lamda floating B- lead AND the ground connection to the existing radios ground will give it a return path for the B+ and ground the case of the Lamda to the radio for safety.
Correct?
Yes, my existing supply doesn't supply as much B+ as I'd like and so I am subbing in the Lambda just for B+ and only temporarily just to see if the extra 70vdc will help performance any. I have an issue with a slightly insensitive S-meter and so perhaps the lack of B+ is affecting IF gain.