I just installed a 28x9x2.75 in mine. IT wouldnt have fit between the rebar support and the A/C consenser if my Radiator mounts wern't messed up from hitting curbs. I'd reccomend a big thin core, like 2.5 or 2.25" x 12 x 24 should be about 28" with tanks on it. If you move your battery over and cut a bit out of the body it's easy to get your discharge pipe through there. So you can get a normal IC not a same side one.
here is my pics and thread.
http://www.newbeetle.org/forums/showthread.php?t=29306