You get a two car garage. Both vehicles must have their engines mounted in front of the passenger compartment. One vehicle must be a 2015 model year or newer, the other must be a 1991 model year or earlier. The catch is that the vehicles swap engine, transmission, and driven wheels.
So if you chose a 2016 Nissan GTR Nismo and a 1970 Chevelle SS 454 LS6, you will have a 600hp, AWD, DCT, TTV6 Chevelle and a RWD, Manual GTR with a 7.4L V8 hammering out 450hp and 500lb-ft.
And no, the vehicles do not trade brakes or tires!