britcon, that link comes up as not found, maybe defunct?
gazim, a 50 series tire will be a bit taller than the 40, and will be taller than the tire you have now. As long as it's not sitting low enough to cause an issue with the tire hight, I don't see that as a problem, however, it will make the speedo read off (slower than actual). As to the backspace issue, what you can do is put some sort of stright edge, preferrably at least the same length as the wheel diameter, on the hub, then measuer back to check for clearance issues. Turn the spindle through its arc to check for obstructions and clearances. Essentially, you'll want to try to keep the rim centered on it's witdth on the spindle, but that's very difficult to get perfect. That's part of the reason newer trucks have such a deep offset. I'm not sure what you have for suspension, but you'll probably end up with around a 4" backspace. I wouldn't think that you would have to chang ethe backspace if you maintain the same rim width, though.
BTW, I mentioned the load capacity from an experience we had at the shop with low profile tires on a truck. Had a customer that wanted a 245/45/16 on a '71 C20, 307 4 speed, 4.56 rear gear. Besides being so short in hight that he almost blew the engine up with too many revs, he kept blowing out sidewalls, The tires just didn't have enough capacity to carry the bare truck, let alone put a load in it. I'm just saying, check the tires specs out, make sure they'll carry the weight of the truck with a nice safety margin, alright. Just want you to be safe!
