Can anybody tell me if a set of 14in wheels from a neon will fit on a '68 valiant that came with a 13x4.5 wheel?

As I recall, the Valient had a small center hole in the rims for the 9" brake drums compared to the newer Valients and Darts with the 10" drums. What you could do is place a piece of cardboard behind the Valient rim and trace the center hole and bolt holes onto it. Then place the template you made on the other rims and see if the holes line up. Good luck to you.
John

Don't the neons also have a heavy negative offset ?

I have seen neons with SBP rallye wheels, so they should bolt on, but i'm bot sure about the backspacing or the offset.

100mmx5 Neon wheel is close enough to go on the 4"x5 A body hub, but it is not recommended. Lug nuts will not seat properly. Attempt this at your own risk and expect to have a huge repair bill if/when you lose a wheel at speed.
If you just want to move up from the 13" wheels, there's plenty of 14"x4.5" A body wheels around cheap. On my '68 V200, I run steelies, 14"x4.5" front, 14"x5.5" rear. Tires are 185x70 front, 215x70 rear. 5.5" wheels are a bit hard to find and run about $100 a pair, 4.5" should go for $10 a pair.

This is not going to be a permanet setup. I just need to put them on to get it home (about 15 mi.). I plan on putting a set of rallyes on it eventually.