Yes I dry fit.
You can stuff cotton in any gaps.

I may ream out more from the Humorous bone and may shorten the Ulna bone but the mouthpiece I leave long(radius bone). I make my calls longer than any I have seen.

Now if you get sound you are happy with you can still make one more improvement.

Take a dab of epoxy and glue the mouthpiece shut. Then when dry drill out with a conical drill bit. I use a Dremel diamond bit. The bit size is 1/16 at the end to 1/8 at the top. This will make the call much easier to use.

JW