Should be okay, I think. Back in the day 8-bit machine only really differed in their PSU input voltage and graphics modes. The NTSC type signal is 420p so US machines were limited to around 400 horizontal lines going down, while PAL units had the full 512 lines on a 576p signal. Because the frame rate is slower on a link that carries exactly the same bandwidth, you can squeeze in more pixels when the clock's running slower. Presumably that means one or other of us wasn't using square pixels (I think it's the europeans as the screen resolution I just looked up was 4x5, not 3x4. That's for a european derived model though (the BBC Micro), and wikipedia suggests european users got the same resolutions as USians with slower speeds and presumably black bars because of the lack of lines going over the slackened RF interface. But then wikipedia doesn't mention the US modes of the BBC micro I know about, so perhaps it just forgot to mention the Euro modes for the C64. It does say the CPU was clocked slower on Euro C64, presumably because they had longer to prepare the next frame for display. On the BBC micro they kept the CPU the same between different markets as far as I can tell, but just cropped the graphics instead.