The simplest, cheapest and most effective option is a 7D.someone else suggested the same to me just the other day!
It will multiply any lens by 1.6 with no loss of aperture speed and better quality than any teleconverter. Cheaper than many of the options being suggested here.
I have the 7d, and use it with all those lenses. Its great as long as light is bright, but as it gets darker, its worthless while my 5D MK II and MK III keep right on going. A 1.4 or 2X TC with the 70-200mm MK II really loses ability to focus except in good light, while the 100-400mmL keeps on going.
There are lots of choices, and every one is a compromise, so pick your poison.