I Ran with a 302-2 pan in my '67 which should be all but identical to your 66. The 302-3 is similar but not quite the same dimensionally and I know if I ran the 302-3 in my 67 with the 1" forward bias blackheart mounts it wouldn't have fit, it would hit the crossmember where it transitions to the sump.
this is the 302-1 and 302-2 comparison
this is the 302-3 according to my google-fu - this pan is deeper where it transitions to the sump and the slope is steeper which would lead me to believe that's where its hitting.
That Chevelle runs a 4L60. I don't know if the L60 is dimensionally the same as the L65 but the tunnel had to be massaged with a mallet to clear the cooler lines. If it were raised up in the tunnel due to a high engine, it wouldn't stand a chance of fitting.
This is how snug my pan is to the crossmember - it doesn't touch anywhere despite the appearance.