My guess as to the reason for such a shallow seat tube and steep head tube is that Eddy (or whoever drew the plans) wanted to keep the wheelbase short for increased maneuverability on a big bike that usually would have a long wheelbase. Luckily for the OP, the shallow seat angle and long top tube might work well for upright handlebars. Back then it was common for custom builders (especially in the Midwest) to make a criterium racing style of bicycle. That was by far the most common type of race here. Those bikes would have steep seat and head angles and a fairly short top tube combined with a long stem. That design of road bicycle does not work well with North Road handlebars.
Back in the 70's when custom framebuilding restarted again in the US, there wasn't a lot of background builders had to make their good choices. That meant they would often color outside of the lines because they didn't have enough experience or knowledge to know what worked well and what didn't. I remember asking Ron Boi (RRB Cycles) where he learned and he said (like many could say) he learned on his own. I'm not sure where Eddy learned either. The father of modern American builders Albert Eisentraut grew up in Chicago before moving to Oakland, CA. Depending on when he told the story of his beginnings, he was either just inspired by Oscar Watyson's in Chicago or actually learned by watching them. Whoever was the Watyson immigrant coming to the States around the turn of the 20th Century brought his knowledge of how to build frames from Belgium. The 4th generation of the original immigrant still runs a bike store in Chicago. Albert taught framebuilding in a variety of 2 week classes throughout the US including one in Chicago. Somehow he had a connection to someone at Turin's I believe.