what rods would be better for my build

The 340 rods may be a little stronger, but they wilol be a little heavier too. This may mean balancing may be a little more. If it were my build, I'd use the factory 318 rod, with good bolts. What pistons are you using? the 340 rods will be bushed for full floating piston pins, while the 318s will be for pressed pin. It costs extra to bush the 318 rod for full floaters, but full floaters are not required in a lot of builds.