Hi Steve,
Welcome to the world of vintage TT's! The size of the hitch ball is probably 2 516", a standard size for trailers of this weight. You should have no problems pulling the Holiday Rambler with your GMC.
One thing I've noticed about the Holiday Ramblers is that they seem to have less liklihood of water leaks or the few that do show up are the ones that didn't. Unfortunately, there seems to be little documentation on the older Holiday trailers as the company was bought and sold and the support information like brochures and owner's manuals seems to have evaporated. It's possible that you'll meet an owner that has some of the product information. The good news is that most of these vintage rigs were built very similar and what was contained in one may be the same as your Holiday. Appliances, for instance, seem to be similar as to year class, across brands of TT's.
Good luck with your project and post some pics up if you have the chance. There are some excelllent restoration threads on the Forum and some really talented DIY'ers so you will have good resources going forward.