Although I've not done this ride on fixed, I've done this ride and a lot of miles on fixed. You might well find 63" very spinny indeed. Something in the high 70s would be my suggestion.
As I've NEVER ridden high 70s for very long, but have ridden standard medium 72-inches for 600s and the like, it will come down to your preference.
If you think of it as gently rolling then you'll need something you can honk up short climbs and spin down the other side, leaving you plenty of legs for the flat stuff — if that's high-70s for you then you've better legs than me
Having 60-inches on the other side would help lift you up to the scenery, twice — well worth it.
The steepest climb on the Llanfair PG 400 is probably on the dedicated Sustrans path between Pensarn and Colwyn Bay, and you'll need to have a good run at that (dog walkers permitting).
That ramp on the Sustrans coastal path always makes me smile — it's short and VERY sharp. If you start in the wrong gear (non-fixies) then you'll be walking by the top. Not wide enough to tack. It's only about 10m up, though