... a body that makes my 120-300 2.8 become a 240-600 2.8 would be a dream.
A camera can't do that. Focal length is an intrinsic property of the lens (unless you believe the lies of Panasonic's marketing department). It's called a 'crop factor' because a smaller sensor is cropping the image, using a smaller portion of the image circle.
A 2x crop sensor with a 120-300/2.8 gives the FoV of a 240-600mm lens on FF, and the DoF of a 240-600mm f/5.6 lens on FF (the crop factor applies to DoF for the same framing). While the lens is always f/2.8 in terms of exposure (which is determined by light per unit area), image noise is determined by the total area of the sensor - the 2x crop sensor at ISO 400 will have the same noise level as a FF sensor at ISO 1600 - that negates the 'crop advantage' of the f/2.8 lens in terms of shutter speed.
The bottom line is that what the crop sensor gives you is more pixels on target (and that's based on the APS-C having a higher pixel density - if you compare the 5DII to the 20D, the crop sensor has no 'reach advantage'). In practice, the FF can give you the same shutter speed with the same noise for the same DoF (when stopping down the lens of FF), but FF gives you the option of a thinner DoF if you want it. If you crop a FF image to the FoV of APS-C (or 2x), at low ISO the IQ is equivalent and at higher ISO the FF will have better IQ. So the real question becomes, how many MP do you need for your desired output? If ~8 MP is enough (prints up to 16x24"), then the 'crop advantage' of APS-C is an illusion and you'd be better off just cropping your FF images in post.