Hey Greg:
I am a little confused as to what you currently have in your car. You mentioned you have a Crane XR700 and a MSD 6AL. Those are both ignition "brain boxes" for lack of better words and its either one or the other. Also, the MSD does not have a flashing light to indicate the pickup. That is only the Crane so I am not really sure which you have in there. Also you mentioned an optical unit and referenced the Crane box. I am assuming you are meaning the optical pickup conversion for the distributor, but you also mention failing points. And again its either one or the other.
I can say that the bosch red coil is most likely not compatible with the MSD 6AL. They list their cannister coils (i.e. MSD Blaster 2) that are compatible with the demands of the high energy multispark MSD 6AL. Crane offers a compatible cannister coil for their multispark Hi6 series box and it is the PS 60 I believe.
An optical sensor replaces the points as the pickup in the distributor and is more accurate. Both the Crane and MSD box will work with the optical sensor. Only the MSD will work with points as the pickup.
On the MSD you do not have to use a ballast resistor as the ignition power is hooked to the box as a relay trigger and not the coil. I know on a Crane Hi6 you don't need the ballast resistor, but I am not sure on the Crane XR700
A final note on these systems. Be sure to not use solid core plug wires. Performance wires are required if you want the boxes to last and the solid core wires will probably burn them out in short notice.
No matter what you have in there here are some suggestions.
1. I would first check or replace the coil next time it fails to fire. This would allow you to rule it out quickly.
2. If you currently have an optical trigger in place in the distributor, with the MSD you can actually hook into the points for an alternate timing pickup. That would allow you to rule out the box. If it never happens again, its a bad optical trigger.
3. If it occurs, and the coil checks out, and you have ruled out the optical sensor, then its the box itself.
Best of luck on getting her back up. I had a Crane Hi6 fail and it was that it failed to pickup the timing signal from the distributor. So I switched to MSD. But they have been known to bite the dust too. They are wonderful when they work right.
Best Regards,
John M