180mm cranks on a 26" just feel so right, if your frame has enough ground clearance. The INB has clearance in spades; you'll love that crankset on that frame...
GT Mowawk hubs look pretty trick; this is going to be a mean INB, for sure....
As for the BB/bearings,
all you needed to run sealed full-size bearings with that crank and that frame is this thing:
https://www.niagaracycle.com/categories/redline-bb-set-for-3-piece-cranks-19mm-american
...or, any American-size BB made for 19mm spindles.
Now, with that threaded adaptor, you need this thing instead:
https://www.niagaracycle.com/categories/redline-bb-set-for-3-piece-cranks-19mm-euro
It'll work, but it'd save $15 and offer more chainline adjustability, PLUS bigger bearings if you just got the American-spec BB from jump....
If you haven't already got a 19mm Euro BB set, it might make the most sense to just get a 19mm American-spec BB and save the threaded adaptor for putting a square-taper crank into a bike with an American shell....