I want be sure that 2007 F250 4x4 Super Duty 5.4 V8 has low torque under load not much power when pulling trailer under a load going up hill. I want to put a bigger Engine something in the range of a 6.1 it should fit without any modifications? Also will the transmission fit 6.1?

 

The other gas engine for 2007 is the 6.8L V10.

Don't forget that it's not just swapping engines like the old days. You also have to deal with differences in the wiring harness's for the various sensors the motors have and also the PCM, which may be VIN locked.

And swapping in the diesel 6.4L Powerstroke is a bigger change...