The 91 is open.
As far as "air pockets" in the closed system, I've really beaten this one to death. Properly maintained, there's NOTHING wrong with the closed setup - otherwise, why would more and more OEM's be using it?
If you never want to worry about air pockets again, simply drill two 1/16" holes in the thermostat flange, and install with one at 12:00 and the other at 6:00. Et viola! a self-air-bleeding system.
This is a good idea on pretty much anything with a thermostat.
The common consensus is that people who note a significant "improvement" after the conversion were probably coming due for a new radiator only - I've talked to a few ChryCo/Jeep techs, and the OEM radiators (all the way up to 1995 or so) were good for about 150K before they wanted replacing.
5-90