ShermanT48 track and extenders

you get the duckbills in the AFV Club track set

The new DML Firefly comes with T-48 tracks and extenders. You might want to pick this kit up and do a little track swapping.
If not then Major Rob's suggestion is what you'll need. I believe the extenders from RHPS will only work with their tracks, so you'd have to order both the tracks and extenders from them.

RHPS makes a set of duckbill extenders.