Just so you know...I believe you can still rent the Isky Oring tool for a good price. I've done hundreds of cylinders with one as the shop I worked at at that time owned one.

Also, you need to Oring the heads as well. You need a receiver groove. As a policy, I always ran the Oring in the head groove if it was aluminum and used the groove in the block as the reciever. We did this because the materiel between the cylinders can get very thin with grooves through them. Without the wore in them, the would fail. Then you'd have to weld everything up, machine the heads flat and Oring them again. The wire in the head helps support those thin areas and they will take more abuse before they fail.