I can't help you on tire sizes but I can tell you a '56 is not a second series pickup. There were two different series of trucks sold in 1955. All '56s had the same cab as the '55 second series but they are not considered second series trucks.
Cletis: I do not understand exactly what you are trying to say? I have always referred to them same as Chevy, 2nd series. GMC called the release of the new body "Blue Chip", Chevy called it "Task Force". I think it's OK to call 56GMC a second series. In our own 55-59 Gallery we lump them together. Collectors refer to them all as Second Series. It's all good and correct. GMC released the Blue Chip in March 1955.
GMC rod: On stock 1/2T TF trucks, 235/75/R15 is about same size as was used in the day. Maybe a person can get exact tires but probably are a "$$pecialty" tire. As far as your truck goes, different chassis, lowered and you want different size front and rear? We can't guess what would work on a hybrid. We don't know your clearances in wheel well or width of axles or where axles are in wheel well or the look you want and "Lowered" is a relative word.
