Very little interchanges on 39 buicks- different front sheetmetal from 38 and 40, rear fenders will interchange with 39 olds and pontiac but gas filler differs.
Chevy parts are not interchangable. Sometimes seems parts are difficult to
find and pricey- othertimes I find no one else wants them and they are cheap!
I just bought a nice hood side panel for my 39 at a large swapmeet paid $10
for it- saw mechanism for the panel on e-bay at a starting bid of $50 and the
one attached to mine looked to be better.