To me, the sentences are quite different.
"I didn't go so far as..." means that you think calling him a liar is a BIG, IMPORTANT thing.
"I suggested that he might have made a mistake, but I didn't go so far as to call him a liar. I didn't want to upset him."
"I didn't so much as..." means that you think calling him a liar is a SMALL UNIMPORTANT thing.
"I don't know why he is mad at me. I could have called him a fraud, a hypocrite, or a hundred other really awful words. But I just suggested he was mistaken. I didn't even so much as call him a liar."