is this suitable for setting stone panels 20 .inx25 .in x 1 3/8 .in thick over a scratch coat soffit system?

We checked with our Laticrete Rep on this one and they would suggest the use of a cement based thin-set like 254 Platinum to do a full spread type of installation. You would need to put a brown coat over the scratch to smooth it out before proceeding. Please let us know if you have any further questions.