Many stock trucks have four hard lines along the RH frame rail... Very early squarebody trucks might not have the fuel vapor and/or the fuel return line. Return was optional in 1973-1974... It became standard sometime after 1975. The charcoal fuel vapor cans were standard on 1/2 tons in 1974 or a little later and slowly pushed out to the heavier GVWR trucks between 1975 & 1981.
The smallest hard line is likely the brakes. There are not two brake lines to the rear. You'll find the end at the axle hose. The rear axle hose tees at the diff cover to two hardlines clipped to the rear axle.
The fuel system will usually have three lines. Supply, Return, and Vapor.
Those old cracked and splitting fuel hoses are probably SAE J30R7 hose. SAE J30R9 replacement hoses will last longer if the proper clamps are used.
Put those small worm clamps in the bin along with those wasted hoses. They cut into the hose so they loosen up and leak. Breeze Liner doesn't make clamps smaller than 5/8". For smaller hoses like these fuel lines... I use German Fuel Injection clamps or Constant Tension Band Clamps. You could use Oetiker Ear Clamps but they are not re-usable.
My tech post on SAE hoses and hose clamps on one of the other 73-87 boards...
http://67-72chevytrucks.com/vboard/showthread.php?t=696742
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