What's the difference between 7027156 and 7027157. And what application are they for.

 

Both are big block applications (400 and 425)

'156 is the carb for 49-state applications
'157 is for California emissions with A.I.R.

The only difference is the primary metering rods.
'156 used 7031844 (stamped "44") rods
'157 used 7011845 (stamped "45") rods
Note that 67 W30 manual trans cars used '156 with 7031841 rods.