Most, if not all of the long arm kits come with some sort of belly pan. Reason being the long arms attach right above the pan. Most, if not all, use some sort of sub-frame that the arms mount to, and then the pan/trans mount mounts to that.
The pan needs to be removable in order to service the trans/t-case and clutch.
Depending on what long arm kit you want to end up with, make sure what you do meanwhile will be able to come off - or at least be usable for it.
Just the other day we were looking at a Fab-Tech long arm on a friend's Ruby - pure junk! We were looking to see how much trouble it would be to convert it to a RE LA.
The long arm Fab-Tech arms on his are not as long as the RE, and they mount a little different. It uses the stock belly pan with a sub frame.
The reason we were looking to convert it to something better is the stock pan has bent upwards so far the T-case was jammed against the floor and the shifter was so out of position it would no longer go into 4wd. (The stock pans, or shovels as they are lovingly called, are really weak!)
Plus EVERY arm on the Fab-Tech is bent badly, fronts and rears, both uppers and lowers. Plus he's broken I think 4 of their rear track bars, and one front.
A while ago he replaced the track bars with RE track bars, no problem since with track bars breaking, but -.
I don't like the way the sub frame is - looks very weak - the entire suspension depends on it.
We haven't done much yet except replace the stock belly pan with another - after I really strengthened it.
Just think ahead, talk to folks that really use them, not mall crawlers. Make sure that things are reversible if you decide you don't like it after all. Welding on the frame can really weaken it if not done correctly, and they are hard to reverse.