You may have plenty experience with this, but I will post anyway for anyone that may read it but not know some SOPs.
How big are the 13 calves? That will determine your load. Also, make sure to haul all the pairs in pairs and not split them overnight. Depending on size of calves, you may want to load the calves first so you know for sure which cows to take the first day. It might be that you leave the tamest pair(s) back for the next day.
Also hope your replacement heifers are tame, as loading on different days may make them hard to pen two days in a row if you can't keep them penned overnight. If they were last year's calves you held back, I would have expected them to be bigger than 600, which would make for tighter loads.
On that long of haul, I would be sure to use your divider gates and check your load every once in a while. When I was 18 I had a cow go down hauling about 180 miles. If you put too many on, it can be a real pain in the ass to get them back up, even with a hot shot. If I remember right I had a mix of 9 Jerseys and Holsteins on a 20x6 GN. A hot shot and a set of nose tongs (on a good chain or rope) are great tools to take with you.
If you have a big enough truck, I would sure keep trying to find someone local (friends of where ever you are taking the cattle) to let you use a bigger trailer.
And make sure the floors in the trailers are good, and lights are working. Hoofs and pavement don't mix (not my story, but heard a bad one).
You aren't saving money, but since this is cheaper than therapy, good luck. In the words of Dr. Hall from A&M, Proper Planning Prevents Piss Poor Performance.
Having a portable gas air compressor, extra spare tires, the right size wrenches, and good jacks may keep anything bad from happening, as it seems the problems always arise when you aren't ready for them.
ETA - and be sure to post how it went afterwards.
[This message has been edited by milkman00 (edited 5/13/2011 12:08p).]