Bottom line: If I had plans for a building/buildings drawn in 1971 (prior to the Architectural Works Copyright Act of 1990), and the said building/buildings were never constructed, can I now build them, 38 years later, without the of the firm who first drew them?

Let me clarify: I have general sketches - not the actual plans. If I were to design plans based off the sketches provided in 1971, would this be an infringement?