Both JTR and the hooker/scarab adapter kits use the same mounting location on the block, as well as the same chevy truck style mounts. The JTR kits has setback plates, as you noted with the pics/etc, whereas the hooker/scarab kit mount the engine farther forward than JTR. If you have a '70 or '71 car (might be '72, as well... depends on the transmission crossmember that's used), the hooker/scarab mounts do set the engine back slightly. For '72 and later, the mounts actually set the engine slightly forward. I had the hooker/scarab kit on my '73 and switched to the JTR setback adapters. The number 1 spark plug was about in line with the strut tower.
I won't be using hooker/scarab mounts again, but I have a friend that has them in two of his cars and has had no issue/complaints. I just prefer the setback position. It takes a little more work to install with JTR, but I think the benefits are worth it.
There are pics in my album linked in my signature showing how my 377 looked with the hooker kit. Pic 11 shows how far an HEI will sit from the firewall. Pics 208 and 218 kind of give an idea how far back it goes with JTR.