The difference in the trucks is 8"
FL112 means it's the 112'BBC and fl120 is 120"BBC
112 is 112"BBC Bumper to back of cab, tighter turning radius, CANNOT get a detroit it doesn't fit
120 is 120" BBC Bumper to back of cab. Can get detroit and it's 8" longer in the hood
The 112 is all you would need and the Mercedes or Cat engine are both great engines.