Its not hard to put a Muncie behind an LS motor; and a Luk Repset clutch is absolutely fine for what you're doing. Best thing you can do is figure out which Muncie you have; Ive posted it all before, or just PM me. If its an M21 or M22; You'll want a 3.73 rear to get your tank moving. At that point you may be better off selling it and saving your pennies. If you're mechanically inclined and have a shop press, you may want to disassemble the trans. If its a stuck pig, planning your build around it will take on another $5-700 easily. I'd say the best bet for you is an M20 (2.52 1st gear), but you're still going to want something like a 3.23 or more rear gear. There is a 2.98:1 1st gear M22 gearset (aftermarket only), you could run 3.08s happily but between that and an overhaul kit, you'll be in it almost a grand in short order (you'll probably need forks, mainshaft reverse, the splined reverse idler and I'd definitely pop for an iron midplate). This is assuming the countershaft bore hasn't had a cobb-job to keep it from leaking.
I love 4 speeds; but Ive seen a lot of junkers and I want you to be realistic. Nothing worse than planning around something, muscling it into place and finding out it in sad shape. Then you gotta tear it out, find the money, fix it and shotgun it back in there.
BTW the Z-bar is the way to go. A lot of hydraulic setups in the aftermarket just don't seem to last long, or work as expected.