Just changed the lines on my 1996 f-250. They are 5/16ths. The only clamp on to the steel lines with a "pinch" on clamp. I changed all the 5/16 rubber lines to 3/8ths for my new cooler. I completely bypassed the radiator cooler since it only warms the fluid. But the 3/8ths line fit onto the 5/16ths steel lines fine with a few clamps.
But your just going to replace some lines? In that case buy around 8ft of 5/16ths transmission line (or be cheap like me and use fuel line) and a hand full of screw tighten style clamps. The smallest ones and you're set. The steel transmission lines come out right behind the radiator horse and right above the front tie rods. You cant miss them, one cures slightly towards the passenger side (this is the return to the transmission) the other curves sharper to the driver side (this is from the transmission, and should be ran into the radiator, then back out into the external cooler.) But if you live in a warm region just bypass the inline radiator cooler since it just passed hot water through it to heat the fluid.
If it has a stock cooler it will look like a power steering fluid cooler (a real joke)
Click for picture
You will wanna upgrade later on. I would recommend a tru-cool 40k cooler located at thedieselsite.com.
But ya all you need to replace the lines on these trucks is about 8foot of 5/16 hose (or 3/8ths with a bigger cooler), and some screw style hose clamps. They use snap on clamps.... there really isn't much pressure on these lines.