My original traveler is failing and I want to replace it with a windward sheeting traveler. Does anyone have a recommendataion for a foolproof solution (e.g. Harken midrange)? I have a later model J/30 with the track in a recess on either side of the cockpit.

Harken Midrange. Checking their web page it appears to be the 1635, rigged 4:1. Many boats have mounted their control blocks on the combings to allow the use of more of the track.
http://www.harkenstore.com/uniface.urd/scpdinw1.ShowProd?409Z56LC4VIK2

If you are a high 400 to 500+ hull number you need the high profile track to span the cockpit. You'll also want to fill in the track to raise the traveler a bit. Some have used wood, but many like ours have an insert of high density plastic, although I have no idea what type.

Don't know if Henry Mallard is still at Layline, but he was always a great source for parts and advice.