73 is likely the width of your BB shell. 126 the length of the shaft. So half of those 4 mm's shorter the one you found might leave your chain wheel rubbing the stay and your front DR not lined up.

It's a BSA threaded shell though so you have a lot of options. Why not just ask your nearest Trek Dealer. Sure you might pay 40 buck instead of 18 bucks, but you'll know you got the thing you need. They might even install it for you for not much.

You can be pretty confident there is something out there that will work.