i would go for a CA18 as they are smaller and easyer to fit , but still have good power
and a CA gearbox will fit in a standard tunnel but you will need to cut a new hole about 100mm back for the shifter
it abit hard to fit the CA box in the standard tunnel but has been done heaps of times ,its just abit of a hassle changing the box or takeing the motor out

as for the SR20 im pretty sure the floor will have to have a bigger tunnel put in
and the shifter will be further back aswell
and ive heard that the SR20s in 1200s are abit front end heavy and dont handle

so i would go for the CA18DET and they can be regoed in 1200s in some states !