Problem is that that looks shit and you get fudge pixels which are not all the same size.
What you have to do to get clean aspect scaleups is find the lowest common multiple of the 2 sides so that you can scale them accordingly.
The least common multiple of 640 and 350 for example is stupidly high, 22400. So that means to get the proper aspect for 640x350 on a LCD with equal sized pixels throughout you would need to scale it to 22400x16800. And if you say, oh but that can be made smaller. like halfing it down to 11200x8400 or even lower. No, then the pixels are not equal again. because 22400/640=35 so halfing it would result in half pixels, which would result in uneven pixel distribution.
We are nowhere near the resolutions needed to scale this stuff 100% correct.