Yes you can get it in, but you will need new motor mounts. Get the Wolfpack radicals conversion kit. It might cost a bit, but it is the easiest way to get a big block motor in the RS4 series cars.
On a different note, don't get a megatech, and don't get a .21. If you want to use a big block, stick to .15's, unless you have some experience. If you want to stick with a .21 then I would suggest an engine from a different company such as OS, novarossi, and RB. Megatech engines unfortunately are all low on the power-to-price ratio even if you are looking at a cheap model.