No, if he's got the trans apart at the builder he can probably save $$$ getting the internal gear done, I missed that part. There are numerous speedo gear charts out there. For 3.73s, you can use a 7 tooth drive gear and a red 21 tooth Ford gear or a 6 tooth drive gear and an 18 tooth yellow Ford gear.
6 tooth is best as you can go all the way up to 4.30s with a simple speedo gear swap and I don't see anyone ever going below 3.27s except road racers. A 7 tooth can only go to 3.73s and I'd personally suggest 4.10s over 3.73s.
3.73s are kind of a bastard gear that everyone likes (me included for a long time). Don't accelerate as fast as 4.10s and its not like there is some massive difference on the highway. NA 2V cars need all the bottom end they can get, even built ones. Don't fear the gear.
Note the blurb about the 23 tooth gear on the bottom. Experienced this firsthand on my first mustang that had 3.73s in a T-5. Speedo stopped working, pulled the gear and it was just nubs. This was after 6 months.
If Zak has it apart, get the 6 tooth gear installed while he is at it.