"...the distal ends of the camshafts have off-set notches. That is, the notch is on the upper half of the "circle" on one, and the lower half of the circle on the other. You could not put a straight piece of steel across the slots. That tool you pointed to just keeps them locked, each offset by approximately 1/8" from the same approximate horizontal line..."

Uhhhh, are you SURE that's the way they're supposed to align, or is that the way your's happen to be right now?

I'm attempting to attach a marked-up copy of your photo so you can see what I mean about the marks. (I'm not 100% sure that what I'm seeing in the picture are the factory marks but they sure look like it.

For the moment I'm assuming you shot the picture with the crank at TDC.

"...original owner cut the timing cover index mark plastic bit off, and put some white-out on the 12 o'clock positions on the cam gears and cover...."

I think your cams are significantly out of time, but hopefully not enough to have caused any damage.

The painted marks appear to be about 60 deg off of the scribes. So with that in mind picture this:

You can make the sprockets align with their timing marks all day long. But there are three different possibilities for indexing the sprockets to their respective cams (because there are three bolts). But only ONE combination will bring each cam around to where its notch on the LH end is horizontal. Each of the "incorrect" combinations will put the notches 120 deg off.

My gut tells me you need to back out the sprocket bolts, get the sprockets so that their "real" marks are at 12:00, secure the sprockets with the IPD tool, then rotate the cams until you hit the combination where the notches are horizontal AND you can insert the bolts.

If what I'm driving at is in fact correct, I don't know if you will be able to correct it with the head in place. You would need to be VERY careful rotating the cams in case a valve were to contact a piston.

(All I get on my computer is a red x for the image I'm trying to insert.)