I want to lower my 68.I bought a 2inch lowering block kit for the back and my rear end guy says it wont work due to the centering pin.What is the best way to lower the rear.Thanks Gene

mono or multi leaf springs?
mono or multileaf perches?
mono or multileaf shock plates?

It has three leaf springs and no u bolts.I dont know much about the car.Thanks Gene

De-arch the springs is the best way.

Add some U bolts, remove any leaf pads, cut off the top locator on the spacer. The lower spring plate will be your only locator, make sure the leaf center bolt is well in the hole in the plate.

Some three leaf springs use a countersunk center bolt on top of the leaf so it won't hit the flat mono perch, - mono perches are more shallow, and have no hole for the center bolt. If yours is of that type, there won't be anything to hold the spacer in place. Swap in a grade 8 bolt or proper leaf center bolt to hold the block. An alternative is to capture the spacer to the housing perch.

Since you say you have no U bolts on there, that tells me it's a mono perch housing. Multi leaf perches have one U bolt per side. Four U bolts is best when you re-do it.
David