You might also look at the 3.3L QSB common rail Cummins. It gets excellect economy but has roughly the same power as the 3.9L 4BT and is a bit lighter.
The 5 cyl mercedes found in Dodge Sprinters are excellect, from what I've heard and are economical, plus they rev a lot higher (about 5k rpm IIRC)
the 4BT has been around forever, and it is a 6BT (5.9 L) minus 2 cylinders, so there is a lot of parts that swap plus the 4bt, being around forver, has myriad available parts and support, and is easier to find trans that fit it easily. While the 6bt is a great engine, so is the 4bt, yet it has the ability to get better economy.
I'd definitely recommend you go the
Cummins forum. Lots of good info and many of the folks are really into economy.
Living in Cali as you do, you need to talk to a CARB ref ASAP before your dreams are smashed.