About 3 feet of extension is what I use. And I prefer a "wobble" end for the socket; when using a universal-joint swivel end I run a wrap of electrical tape around it to make it less "floppy" but still able to swivel.
I have gotten the top two bolts on the OHV from the top with a wrench, but my preferred way to get them is to lower the transmission tail and use the long extensions with a socket and ratchet (or battery impact).
If you're dropping the tail really low, be sure to check the fan-to-shroud clearance, and the distributer cap clearance on the older ones (and that's assuming you've already disconnected the shifter, wiring, and cooling lines on an auto). On a 2.9 you have to watch for the intake hitting the firewall, too; I don't remember it being an issue on the 4.0. Dropping it too low is not nice on the engine mounts, either.
Mine are all 2WD; I use a spare yoke to keep the oil/ATF from running out of the tail shaft housing.