Based on a chart I have left over from my generator project, it looks like you'll be undersized. 70ft of 1/2" will carry 46 CFH. About 1,000 BTU is approx. requires 1 CFH. Your dryer is about 30-40,000 BTU and I'm assuming the Reznor 75 is 75,000 BTU? You also have to factor some losses for...