robot, I hadn't thought about it, but I see what you're saying. When you butt a later head up to the old head to transfer locations, you can get a mirror image. And in addition, like you've shown, the left and right heads are drilled differently.

I think what I'd do is use the accessory brackets, lay 'em on the head (standing on end), locate 'em per the dimensions you've shown in the link and center punch the head through the bolt holes of the brackets. Drill the heads through to the water jackets with a hole large enough to accept long nuts 3/8-16 (long nuts are available at specialty bolt and nut suppliers) and weld the nuts in around the circumference. The long nuts will give you plenty of thread back inside the water jacket even if you grind most of the nut off almost down to the head surface on the outside.

This would have to be a really great set of heads with a lot of money invested in 'em for me to go to this much trouble to save, with the GM L31 heads flowing like they do and being so affordable.