The Isky E4 is fine for the lower compression 2 barrel. Here's the 273 cam specs and the E 4 specs. A little extra duration and lift won't hurt a thing. There have been dozens of members here that have installed a E4 with a 2 barrel. The small carb will just limit your upper rpm.
Basic 273 cam specs:
1965 180 hp 2 barrel .395 /.405 lift 240°/ 240° duration
1965 235 hp 4 barrel .415 /.425 lift 248°/ 248° duration
1966 235 hp 4 barrel .420 /.430 lift 248°/ 248° duration
Isky E4 .425/.425 lift 260°/260° duration 216° @ .050
Here's a link to my 273 build just below.