I asked about the outside dimensions because the 4.5" of Y travel available with my mod isn't enough to cut the outsides of a 4.5" part (because of the diameter of the end mill). So, for that part you'll probably want to go with that 7" toolmakers vise. The other option is to size the stock to exactly 4.5" before putting it into a perpendicularly mounted toolmakers vise.
The slots on the Taig are a little larger than 0.25" wide* with 1" spacing**.
* They're 0.266" in the
3D model someone named Richard Crook made that is available from cartertools.com
** Hmmm... apparently the slots are 1.006" middle to middle on that model... that's odd.