Follow up....
I got the JD 1217 haybine going well, and cut 10 acres pulling it with a JD 5410. It cut great.
I had another 40 acres to cut where the JD 850 was more convenient, and that little rascal did very well. Hydraulics worked fine. The only trouble really is that it will run hot if pushed too hard. I double checked the radiator, which was clean. I changed out the air filter thinking it might be running rich and overheating before it should, and that showed remarkable improvement, though I occassionally ran hot on hills. I cut 3/4 of that hay with the 850, and used it to tedder, rake, and baled over 1200 square bales with a JD 336. It's a gutsy little diesel.
As a side note, the haybine had a catastrophic failure. The reel stopped turning...I was thinking belt, but actually I had been unable to see into the area where the shaft comes through a bushing to the drive pulley, and it had long ago worn through the bushing and into the sheet metal--like a lathe it cut a groove into the shaft and cut the last six inches off in the middle of the hayfield. Luckily, there was a parts machine I knew about just up the road, and I switched out the entire reel, and 24 hours later was back in action. I hope everyone elses 1st cutting was a little less spectacular than mine.