I have older flat top pelpro which I am upgrading to a pp130. The chimney goes straight out the back through the wall outside then up. I didn't install it I bought the house this way. But the new pp130 chimney/exhaust is 6 inches lower than the hole in the wall. What's the best way to make it connect to the existing piping without puting blocks under it to riase the entire stove? Is there a flex pipe or use 2 90 degree elbows?