What about the bullmoose are you trying to approximate? To me, the
bullmoose offers a very solid, non-flexing handlebar (I'm saying that
as a positive), and it's very hard to duplicate that with a bar/stem.
The newer ones from Riv came in 150mm and 200mm; older bullmoose came
in many varieties. I believe the DD only comes in 80mm and 100mm, so
you're not getting anywhere near the extension of a bullmoose. You can
get a pretty rigid bar if you go to the 31.8 size, which I have on my
bikes that don't have a bullmoose, but that usually doesn't fit the
Riv esthetic. If you're just looking for a flat bar/stem combo, what
you're proposing is fine. As to whether you should get longer or
shorter extension, that depends on what fit you are comfortable with.
A longer quill length obviously gives you more height adjustability,
but I can't tell if that's what you need.
jim m
wc ca