Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
How old are your 105 brakes?
If your 105's have recessed mounting bolts, it's a straight bolt-up. Use a 5 mm allen wrench to loosen the 105's and bolt up the Ultegra's in their place. You'll probably need new cables.
If your 105's have mounting bolts that extend all the way through the fork, it's still do-able but it's going to take a bit more work. Drill out just the back of the fork to the diameter of the recessed bolt. You'll need an extra long recessed hex nut and install the REAR brake on the front. Now you can use the front brake, with a conventional nut, on the rear of your bike. (Save the curved washer from the brake that you took off.) It's hard to drill out the seat stay bridge because you don't have enough room for a drill.