The block is centered by the nipple just square it up. The top of the block should also have a nipple for the plate to center on. The plate just sits on top no welding.

 

Almost........the top of the block has a "hole" in it where the leaf spring bolt head sits in it.

The U-Bolt plate has 5 holes in it, one for the leaf spring pack bolt/nut, and the other four in the corners for the U-bolts. The plate is held side to side by the u-bolts and front to rear by the center hole that allows clearance for the nut from the spring pack bolt. The top bracket for the leaf springs is NOT welded to the springs.

I'm not sure if there are blocks on the 2wd trucks or not, but if you hold a straight edge on the bottom of the block you will see that the block is indeed tapered all be it slightly. The problem with taking out the block and having the springs rest on the perch is that the springs are tapered the opposite way as the perch so you have thissort of thing- )(


Here is a pic of a factory block with a straight edge on the tapered part:
Hope that clears this up for you.