Well, from doing a quick search (lol, keep trying.
) I found that one recommended size posted here is about 22" x 12" x 3". I've seen even larger shoe-horned in there and smaller. I think mine was a bit shorter. Really though there's no easy answer for this... You're the one who's going to have to figure out how you want to hang it, how to run the piping, how you want to relocate/mount the oil and power steering coolers. You are probably getting a rough idea why many people opt for a
"Spearco" replica IC at this point...
Universal piping kits aren't bad as they're cheap, but usually the couplers are crap and they often only come with conventional worm-gear type hose clamps. I've found that anything about 2.5" is a bit of a pain in the ass to install due to the kit only having so many angles available. Ordering a couple 'hump' couplers and a couple long couplers will help you get those subtle angles.