[qksilver]

springs are different lengths also but have seen them converted/perverted. Check out Senators posts. With the complete donor vehicle you could use the later model spring with perches on the earlier frame. Not exactly sure, haven't gotten to it yet, but i think the frame widths are about the same. So if you used the later springs and perches on the earlier frame might not have to change the spring mounts(widthwise) on the rear axle.
Drill out the old perches, mark, match and measure a few times then drill new holes, bolt the perches on with grade 8 hardware.
The later model leafs are longer so they will give you more suspension travel and probably better ride depending on the type/condition of the springs, plus easier/cheaper to get all spring lifts which are always better than blocks for the 2-8 inch lifts