I believe you need the gross vehicle weight not the curb weight when considering new tires. In other words what will the truck weigh fully loaded, not empty.

This chart from the site maintained by Keith Hardy will give you the info for the original vehicle. I don't think your updated engine and tranny should change the numbers that much.

Hope this helps.

http://chevy.tocmp.com/1948_51truckshop/51ctsmA4.htm

"Nothing astonishes men so much as common sense and plain dealing." - Ralph Waldo Emerson