Thanks to
WVZR-1 who sent me his old LT1 cluster board, AND THIS REGISTRY SITE, I was able to harvest the differential op amp from the LT1 board and install it on my ZR-1 cluster. Thanks very much, Dave!
Once I had the "new" chip installed, it only took a couple minutes to get the tach to track the rpm on my scanner. But, I thought I would confirm what I and others thought might be the case - the compatibility question re using the LT1 cluster as a potential source for parts. From measurements I did, all the components on the LT1 cluster are the same values as the ZR-1. Hope that helps.
Anyway....I just wanted to thank everyone that responded to my call for help. Once again the "Brotherhood" comes thru. Owning a ZR-1 without this site and the people here would be a whole different experience when things go wrong!
P.