We have used Twin River Outfitters a few times and they are great. They operate out of Buchanan and offer trips and rentals from Iron Gate to Balcony Falls and have private campgrounds along the way. This past Memorial Day, a group of us floated Iron Gate to Buchanan (35.5 miles) over 3 days - 2 nights, camping both at TRO sites. The sites were nice and chopped wood was plentiful. Over labor day a few years ago, we rented canoes from TRO for one-night trip from Buchanan to Glasgow and DIY-camped on a small rocky island. Overall, you can't go wrong with any of those stretches. But 10+ miles/day on that river at summer low flows, although amazing and beautiful, is definitely more paddling than fishing. Try to keep it in the 5-8 miles/day range if you want to get in quality fishing time. Or even 3-5 miles if you want a shorter second day. If you're renting, go with a canoe so one person can steer/control the boat while the other fishes.
Here's a great resource for the upper James: