any help is greatly appreciated!

 

Prior to 1987, SBC were flat tappet lifters. In1987, all GM sb's used roller lifters and they use a keeper in the lifter valley to keep them from rotating. Each pair of 2 lifters has a plate to maintain direction of the lifters and there is also the large retainer in the lifter valley, what I call the spider that bolts in to keep the lifter retainers from moving.

Therefore the 87 and up blocks are machined to accept the 8 lifter keepers and the large (spider) retainer. In addition, since the roller lifters are longer than the flat tappet lifters, the push rod length was reduced.

I tried to find a pic but I couldn't find one.