Yes and No.
The top is flat, so the radiator should fit as long as some of the holes line up.
However, then you'd need to mod the TOP PANEL so the hoses can reach. Something like a dremel tool.
The 760D allows for 3x120mm, or 2x140mm radiators on the inside so the 420 is too large there as you know.
Thus, you'd want to do THIS:
1) 2x140mm on inside of case screwing into radiator
2) radiator on top
3) drill holes into case TOP so hoses can reach.
You could alternatively put the 3x140mm fans on top but that's really messy, dangerous, and the 3rd fan might not help much.