I'm not sure your idea of "minimal modding". I'm not sure the top would fit a 420mm with my idea of minimal modding, I have a 360mm there and I don't think there is room for much more, if you did mount one there it would likely have blocked air flow over part of it without what I would consider more than minimal modding.
The case appears to support 2 420s out of the box in the front and right locations. Just may need to very careful about fan radiator stacks on both locations to they don't impact each other. But I think as long as you keep the front fans where they are it should be ok. If wanting a pull fan setup would just need to put the front rad where the current fans are and the fans just inside the case from the front rad. Depending on fitting locations on the radiators you might also have one of them have the rad hoses top while the other is bottom to prevent interference there, I would not know for sure on that without some test fitting. That could make the fill/bleed process very annoying. I just think the right side would have limited air flow without moving/replacing the glass side behind it.
I have the Corsair H150i 360mm rad in the top, I have the fans included with that kit as push fans under the rad with the rad and fans mounted on the inner side of the case. I also have the case's 200mm RGB fans between the case and the top filter as a pull fan set. The rad is 27mm thick while the rad kit fans are 25mm and they are almost even with the top of the Asus Crosshair VIII Hero board I have in the case.
I've been considering a custom water cooling kit with this case, but with the setup I would want on the GPU and CPU/Mobo with a little over kill on the rads, 360 top as first rad that will get rid of most heat to outside the case and then a 420 front with the room ambient temp getting pulled in to cool it more. But, fittings, pump, res, rads and hose I'm at over $1,000USD and I'm not having a problem with my temps right now, so it would be more for just noise and looks. I know that if I get a second GPU I will have to go water cooling on them since there is not room for 2 vertically mounted GPU cards in this case with each needing 3 rear slots. That second GPU would add about $400 to the cooling setup. But, no second GPU right now at these prices, took my over 3 months to get the first one while actively trying to get it.
I've also thought about remote radiators outside the house to remove the heat from the room all together especially in the summer, but I think that adds another $600-700 for additional rad, hose and quick disconnects. Quick disconnects would allow my to bypass the outside radiator in the winter when I would want the heat in the house too. Also, the Level 20 GT case would need modding to get the hoses outside it, unlike my older Level 10 GT that has the 3 hose ports in the back of the case.