I was down that way in March. I prefer the Laredo crossing - use bridge II at the terminus of I-35. After crossing the bridge you'll want to stop and do paperwork in the complex which is almost under the very bridge you just crossed. Bear left and watch for a sign indicating a very sharp left turn. Drive back towards the bridge, get your permits and then backtrack to the same road you made the sharp left turn on. This time turn right and head towards Monterrey toll or free - both roads are good. Watch for a right turn as you enter Monterrey - this toll road avoids Monterrey and will connect to 40. 40 west is good either free or toll. Before Saltillo look for 57, take it. 57 can have some surprisingly heavy traffic and can be slow at times - other times it is fast. 57 is a good road, no topes and much like a highway in the US.