Thanks for all your replies!

I'm actually new to bicycle repair. So I've rebuilt the rear wheel once (with some transplanted spokes) but the spokes were 2.0mm vs 1.8mm (original). So I disassembled again, and ordered some 1.8mm spokes.
What I learned (from the rebuild) is that lateral truing is off by 1~2mm on both sides (it wobbles slightly). But also, I think there's one section that needs radial truing.

I actually picked up an extra set of wheels recently (visit to Montreal). Rigida 27x1-1/4. But they are 4-cross laced (spokes are 307mm length), high-flange hubs (noname). The rear wheel has english threading (and I need french).
But also, the front hub is 100mm o.l.d which I learned takes a 3/8 inch axle. My Motobecane front dropouts takes a 5/16 inch axle.

I intended to use the extra rims to build new wheels. But I actually want to save the originals and rebuild them (and also buy new hubs for the extra rims). For the learning experience (as well as a set of backup wheels).

So going back to your replies [MENTION=397345]Classtime[/MENTION] says put the good rim on the rear, while [MENTION=45088]nlerner[/MENTION] says put the good rim on the front. I suppose it doesn't make a huge difference if the wobble is only slight?
Thanks.

Note:
Original front hub is Weco high flange on Rigida rim.
Original rear hub is Atom low flange on Rigida rim.