wow you guys have overthought this big time. He has the wrong equipment for the job. As others said, this was a joke of a century regarding elevation gain. The problem is the combo of incredible endurance AND hills. And sometimes a ride with lots of short steep climbs is more brutal than long gradual climbs because you can't help but have to go into "the red" a little more on a real short steep climb. People don't realize how much that really does burn you out early on. Do you ever see marathoners suddenly do a 50 yard dash in the middle of a marathon? If they did that just a few times they would Fry themselves and stand no chance of finishing in a good time.
In my opinion almost anyone over 200 lbs probably should have a compact. If you 200lbs on the button you might have an 11-21 casette but you would be hard pressed to tell me that your going to spin out of that all that often unless you have wide open downhills all over the place and even still a 50/11 will get you 35+mph before you spin out of it. IF your century is moderate than a compact with 11/28 should be good and if it's really hilly you may even want a 32 cog.
The other thing that is important is to always use your largest cog on every significant hill right from the beginning. Don't start out and say "I feel good so I'm gonna mash up this hill right away" keep it easy and conserve so that when you get late in the century you can increase your intensity if your still feeling strong.
OP, you psyched yourself out. nothing in your post tells me that there was any reason for you to stop, you chose to quit rather than press on. Chafing should have been a non issue, if you had done 70+ miles in the past you should have had that prepared. Clydes also make way too much out of nutrition. first of all, most of us have tons of excess body fat so as long as we take in a consistent supply of sugar we will be able to burn some of that fat as well but fat burns in carbohydrate flame. All you need to do on long rides is eat your normal meals and then set a target calorie mark to take in each hour. My target is 300 calories an hour. My organized centuries have had rest stops that worked out to be about 2 hours apart so I make sure I start out with 300 calories per bottle then at the first rest stop i eat 300 calories to cover the next hour and fill 1 bottle with 300 calories of HEED to cover the 2nd hour. The 2nd bottle than becomes strctly an electrolyte bottle with water and NUUN tablets. This has worked flawlessly for me for 4 rides of 75-112 miles.
do yourself a favor and get that compact crank. I've seen SRAM Force and Rival as well as Ultegra compacts for under 200 recently