But be very bloody careful when you're going to increase your leverage on that screw! DO NOT YANK!
It's not a typical "solid" screw - it's drilled along most of its length, and it's cross-drilled about an inch from the head. Breaking the thing off usually happens where it's cross-drilled (you can pull the adapter off and still get to the shank on the screw. Don't ask how I know...) but if you get happy and start yanking on a cheater lever, you're probably going to break the damned thing.
Then, it will rapidly move to the top of your list of pisser-offers and make you say evil words in many languages (I think I hit eight or nine when I broke mine, including Arabic, Farsi, and Russian. All wonderful languages to swear in, by the by...)
This is not to discourage the use of anything to increase leverage - it's about the only way you're going to get the thing broken loose. What I'm trying to say is that, if you do use a longer lever, make sure you pull gradually and smoothly if you want to remove the screw in useful condition. (On my RENIX, I was able to use "ganged" combination wrenches to get enough pull on the thing, but I'm a big guy and had a good deal more raw pounds to use when I get there. And I know how to use them when I need to. You may have to use one hand to pull on the lever and one to push on the chassis, or both hands on the lever and one foot to brace against the chassis - but do not yank! A smooth, slow pull is going to get you better results...)
Tips for reinstallation:
1) This is one of a short handful of places I'll say to use never-seez on screw threads. Apply liberally, and torque (yes, use a torque wrench!) to 30 pound-feet. Yes, it's a bit less than half of the spec, but it will make the job easier to do the next time. Plumber's PTFE paste will also work - but torque to 50 pound-feet if you use that (that's still a bit less than what the spec would say, with modification for lube, but it's worked for me so far. Spec on that screw is 75 pound-feet - which is just silly, y'ask me.)
2) Use assembly lube liberally on the o-rings. While most of it will get washed away, some will also get trapped between the o-rings and the adapter shell, and that helps as well. If you don't have engine assembly lube (and why for not?) you can use never-seez. Chassis grease is a third choice - but the other two should work better.
3) Apply a light coat of RTV under the flange at the head of the screw. If you're going to scuff an o-ring putting it back together, this is where you're going to do it - the RTV will help lube the o-ring as you tighten the screw down, it will help it seal once the screw is tight and the stuff sets, and it actually will help you just a bit when you take it off. Second choice? Use plumber's PTFE paste. Follow torque specs in #1.
4) Just like taking that screw out, do not yank when you're tightening it back up! Once you've got the screw out, you'll see why you don't want to jerk on the thing to get it back in place - between the lengthwise shank drill and the cross-drilling, it's just begging to break. I honestly don't know why they say 75 pound-feet for the thing from the off, I certainly don't torque it that far! While I normally follow FSM directions nearly to the letter, that's just one I can't see doing. Someone wasn't thinking that day, methinks - and it's caused plenty of work for mechanics afterwards (I've talked to a few pro Jeep wrenches, and they agree with me. Whoever thought that 75 pound-feet was a good spec for that screw should be hauled out and beaten publicly. They don't follow FSM spec, either.) Follow torque specs in #1.
This is already a painful job - no reason to make it worse.