I'm not sure about the Williams Creek Reservoir access area, never been there, but I have been to the Vallecito Reservoir and you can gain access to the Weimunuche from there. There is a hunting guide located there that may or may not rent horses. If you're not well versed in "horse" you may want to leave them to the experts. They can ruin a hunt real quick if you don't know what your doing. This is a great area to hunt, your going about it the right way in starting your planning early! Good luck!
One other thing, I'm assuming you're talking about elk hunting, GMU 751 is an unlimited archery unit, so you do not need to enter in the draw, just buy the tag at Walmart. 2007 season is from Aug. 25-Sept. 23. You do need to enter the draw for deer.