a longer barrel will give you more speed up to a point. it will not nessacarly give you more accuracy. infact it might hurt it. A short heaver barrel will give you more consistancy ( genreraly) as the barrel heats up.
keep in mind when you start changing one thing on these guns to get accuracy or a differant calibur it sometimes means you will have to change almost every thing by the time your done.
a 308 Saiga is about $500 these days and that should be considered when you are contimplating a build. I have several Sagias I bought just to convert because they should be able to handel more pressure than a standard AK bolt. and they were reasonably priced around $350 . I can build for that kind of money. of course ill have to stick a better barrel and other stuff on the saiga to make it a usable gun for what I do.
I do it because I like making the AK with its poor accuracy reputation shoot better than most rifles I see at the range and piss people off. there ather guns out there that will give you a 308 easier.
if you want a good combination of handeling and accuracy consider a 20 or 22 inch barrel with some meat on it. that 308 is going to bark and kick so adding some weioght and barrel length over 16" will not hurt.
the loss of speed of 18" vs a 26" barrel will only be really noticable at very long range. at 300 yards and less it likely wont even be noticable.
I have a 19" heavy fluted 243 win barrel going on a build that i have high hopes for its reasonably light with he fluting an heavy enough calibur to hunt with and about 1500 FPS faster than a 7.62x39 im hoping the larger than standard OD combined with the weight savings of the fluting will give me the accuracy Im hoping for.