I did 9000 miles in 3 weeks in a '65 544 (B-18 with M41) in 1974. From Rhode Island across the US to Oregon, up into western Canada, back into the US in Montana, and back to the east coast. I took lots of spare parts (fuel pump, ignition stuff, fan belts, etc. etc). The only problems I had were a broken headlight thanks to a truck I was following, broken muffler hanger (fixed with a coat hanger), and a wiper arm that got torn off in a thunderstorm.

I would agree with the other posts that ignition parts, fan belt(s), water pump and the like are good ideas. There are lots of things that can go wrong with a car that won't disable you, or that can be fixed by the closest corner gas station. However, when some essential part breaks, its nice if you have the spare even if you need someone else to install it.