Mostly, time.
Time, space, and motivation are in short supply. (Lots of stuff going on at work. Don't really feel like taking on a new project, until I see what the cretins and bean counters have in store.)
We were talking about moving to west GA (where I'd have room to build a shop) but that's kind of on hold, for the interim. (Don't really have the money to do that, either.) I really wanted to wait until we got moved, to start a swap.
SD4 isn't likely to happen, unless I find an iron block and head SD4 engine. They seem to be even more uncommon than the aluminum stuff.
I really should have kept the engine that was in the Automoda, but that would have meant selling the car as a "roller", swapping something else in (which I didn't feel like doing) or parting it out. (Heresy on many levels.)
Meanwhile, the Duke in the coupe runs just fine, for now. So I'm not in a huge hurry.