If the spot is for standard-definition 4x3 broadcast, you'll have to letterbox it. I presume it's in the can.
If so, it's just a quick trip to After Effects, where you can scale the footage to fit a PAL DV widescreen comp side-to side. Hold keyframes will help you correct for vertical positioning on cuts. Then simply nest the comp inside a 4x3 PAL SD comp, scale it to fit side-to-side, and poof! Perfectly letterboxed 16x9 video inside a 4x3 screen.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA