Don't give up on the tranny yet. If it isn't throwing codes, it's probably OK. You just changed a major component, so maybe it needs a reset (yeah, I'm reaching). It may also need a filter service (external, control main spin-on filter --DO NOT forget the magnet!), so do that. Disconnect both batteries and ground a Batt+ cable to chassis/ground for 30+ minutes. This will clear any previous conditions/preventions. Make sure you're parked on level ground, check fluid level again (the "cold" level is approximate, check again when it's hot). Reconnect the batteries and try again. Restart several times if it doesn't improve. Check again for tranny codes.