On just about anything,you can get away with going up one size without clearance problems;I believe it's a fail-safe from the factory in case,for whatever reason,you can't find replacements in the same size you currently have. On your '40,I'd definitely go for the 7.50's,in fact I'd ask about trying 8.25's on it to see what clearances look like. Around here the tire shops don't generally mind mounting up a tire to check fit and fender,spring and steering clearance before you open your wallet.
When the opportunity presents itself,I try to up-size when I have to replace tires. I didn't on my 2 ton simply because I didn't have the money to jump to 9.00's. I DO,however,have a 1-1/2 ton that can use these new 8.25's if I'm able to upgrade to 9.00 Radials in the future.
My thinking is you can "gear-up" your truck a bit by going up a size or two,here's a calculator to help you figure out what difference a size change will make.
http://www.rocky-road.com/calculator.html This may be the way to bring you closer to what your goal is-better road speed at cruising rpm,or lower rpm at the same speed,but may lose some pulling power if you're running a smaller engine,but you can build a little more power (carbs/intake,headers,cam,higher compression,trick ignition,etc.),probably cheaper than you can build taller gearing(buying another set of tires in a bigger size or re-gearing a big truck rear end). Tire size is a trade off,so think it through and decide what's most important to your driving style before you buy,higher road speed,lower cost,more load rating,lower rpm on the road,more pulling power,better ride quality or even availability,should you rip out a sidewall or throw tread on a Saturday evening,hundreds of miles from home. (Not that something like THAT ever happens to US.
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Speed
