If you get through the spring storms in the midwest, Pueblo is certainly do-able that early. The problem can be between Salida and Telluride CO. I spent 2 years in that vicinity and some of the passes would still get snow the first week of June. They clear US 50 pretty quickly but you still may have a situation where you have to sit out a storm for a couple of days in a motel or your tent. When I last rode across the country(west to east) we rode US 160 from Cortez(just south of Delores) to Walsenberg(sp) Co. Then we followed SR 10 and 167 to Fowler CO missing Pueblo altogether. Still a few mountain passes on that route but less of a chance of snow in late May.