Hi,
I believe a 1/2-ton truck from the early or mid '50's would have either the 218 cid or 230 cid engine. I don't believe it would have the 251. I have a '49 Power Wagon (1-ton, 4x4) and it has the 230 (stock engine for that year), and it kept that engine for the next 12 years or so.
As for the 251/265 being more suitable for souping up, that may be the case due to better construction or design, but I believe it's much more difficult to find performance parts for those engines than it is for the smaller ones, like the 230. Dual carb intakes and performance camshafts can be readily found for the 230, but I don't believe that's the case for the larger flatheads.
Maybe someone else can provide some enlightenment on this.
Matt
