SR20 will happily use a 600x300x76 intercooler and this is what alot of kits for the S13 - S15 and GTiR use (or atleast did when I had my GTiR) Im a little out of date however
The size wasn't over the top on the gtir and despite it adding alot of extra piping that did not translate into additional noticable lag compared to the top mount with shorter piping and smaller intercooler volume
with 1600 the limit appears to be 450x300x76 for intercooler and maybe 180B has a little more space to get a 500x300x76 intercooler once you factor in the overall width including end tanks and enough space to get the 90 degree turns off the end tanks. I would aim for the biggest cooler you can fit given that size constraints with a 180b is going to bes less than S13 - 15 and GTiR and a slightly wider core will allow for a slightly wider radiator core and slightly more airflow
I used a generic 450x300x76 cooler on my 1600 and then supplied dimensions for a radiator to PWR based on the core size of the cooler and my preferred mounting, hose and cap setup. this means the radiator core width matched the intercooler core width
This all fits (just!) behind all the stock grill and trims etc, exposed intercoolers are so getto, so aim to hide it all
however haven't got my project running so I can't vouch for how good my setup is
*EDIT also if you look at the plasman coolers they do a super tight radius inlet and outlet design, while expensive it would significantly reduce cutting required to fit a cooler or increase the width that will fit
swanny