You can raise the angle a little by using the upper holes on the Trans Cross Member.
When I dropped a 396 in my '62, I just used the same 283 Engine Frame Mounts.
Somewhat of an angle is normal.
I did have to change the Oil Pan as the one that came with the engine would not clear.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America