I want to know how to determine what size six cyl engine my truck has in it. It is a 1956 Chevy 6500 with a six cylinder motor. I want to determine if it is original and what size it is. The data plate gives a GVW of 16,000 lbs. and lists the horsepower ratings for both the 235 and the 261. What numbers do I need to look for to find the size and the year? Thanks for the help.