Looks like the old bb spindle was quite a bit shorter. Get a decent micrometer and measure it, or have a shop do it for you.
Unless the old bb is really messed up, I would clean it really good, install new loose ball bearings and grease, and install it. But first, I would take some good measurements on your bike as is.
Doing your own work on a bike has the benefit that you can do such changes without running up a repair bill.
UN26 is bottom of the line Shimano BB by the way (I have bought them before).