The DSD440 is unknown for PDP-8. I've always seen
them on PDP-11s.
I've seen DSD floppies on the PDP-8 (have one, others on ePay).
The best and easiest disk for PDP-8 is a RX01 (or
RX02) Floppy drive
and RX8E board. Thats nice as it was supported and easy hook up
and repair.
Agreed, if you can find an RX8E.
Even if you don't have an RX8E its a trivial
board
easily copied.
I don't know about trivial (I am trying to do one). There are a lot of
unobtainium parts in an RX8E that have to be designed out of a modern clone.
(It's easily doable, but not trivial.)
The same could be said for most of the Omnibus boards, actually.
Vince