I have done it both ways - redrilled the hole in the perch to move the axle, as well as redrilling the center pin hole in the springs.
I prefer not to redrill the main leaf. I feel more secure not doing so since you must rely on that leaf for axle location as well as suspension. I do not have a problem re-drilling aux. leaves - if one breaks you will sag a little, but you can still get home. I also prefer to only drill the holes so that the old hole stays under the spring pad and U bolt plate. I feel this adds a bit of security in that that area of the spring is not seeing near the flex forces the rest of the spring sees.
My last 3 sets of front springs and one of my recent rear spring sets I built all used redrilled leaves, and re-drilling the axle perches is a common practice on Toyota front axles.
If it is possible I suggest re-drilling the sprin perch instead. You will aso need to re-drill the hole for the center pin in the U bolt plate the same distance offset, so don't forget that.
As for drilling the holes in spring steel - I have found that the best method is to use a CHEAP masonary bit , do some sharpening of it with a grinder, and use plenty of cutting oil. I have used this method with both drill presses and hand drills - it works and works well [img]/wwwthreads_images/icons/smile.gif[/img]
I did not know plates like the ones shown are available, but something like that looks very simple to make. Be especially careful when using free floating plates like that though (same applied for free floating degree shims) and make sure your U bolts and plates are torqued to spec - the extra pieces increase your chances of the axle moving around on the spring if they were to break or come loose...
Good luck [img]/wwwthreads_images/icons/smile.gif[/img]