Check to make sure the engine is level from side-to-side. If the engine is not installed level, the headers will be low on one side and high on the other. The best way I've found for checking this is to place a 4' level on the
oil pan and measure each side's space between the level and the frame. A small difference in level translates to a large difference once you get out to the headers.
I recommend checking this first.
I learned this the hard way, incidentally. I installed my first engine and noticed the
oil pan looked slightly crooked relative to the frame---not by much, but enough for me to notice it. I had to loosen the mounts and lift the engine and straighten it out. I was much more careful when I later installed my new KC 331.
DD