would it still be advisable for me to
seek out some mountain flying instruction?

To answer your question directly, I think based on what you have described
that if you take time to flight plan rigorously, try to stay day VFR, and
read a good mountain flying book like Sparky Imerson's you'll be good to go
without specific mountain flying instruction. That said, getting some extra
instructional time in new conditions is always a good thing, but I think in
your case not strictly required if you study up.

Somebody here posted recently what I think is great advice. Night,
Mountains, Single Engine: pick any two.