The original ecm had several ac inputs. One from the ac switch on the dash, plus the low pressure cycling switch on the accumulator, plus a high pressure switch on the manifold by the compressor, plus another high pressure safety in the back of the compressor. When everything is right, the ecm fires the ac clutch relay in the fuse/relay box under the hood. Is the ecm for the new engine a gm unit? If so it should be able to run the ac like the 97 did. If nothing else, you could take the dash switch signal wire, run it through the accumulator lp cycling switch, and run it to the compressor relay directly.