Since he's relocated his springs 3" per side, those spacers won't do him any good. He'll either have to drill new holes rearward in the front relocation boxes, or drill out the spring perches like he asked about. I ended up simply drilling out a new set of holes 0.75" rearward in the front relocation boxes in my 66 Dart.
If you redrill the spring perches on the axle, you'll have to do the same to the lower shock mount brackets as well so the u-bolt holes line up. Depending on how far you move the axle back, keep an eye on shock clearance to the axle tube too. Of course, make sure you've got enough slip yoke engagement in the transmission.
