I have no doubt you will be fine with 7.62 x 39. I can find to buy more 7.62 then I can 5.56 x45 (.223) and cheaper too. 308 Is more expensive then either of those. Plus if you have to run carrying 1000 rds of 308 versus the 762 will be a little easier, not much though. I like 5.56 for that very reason. I can carry about 300 rds loaded in mags plus a pack and still move. One 308 = three 5.56 in weight.
Buy your ammo with 123 gr. bullets or lighter if you find them. If you reload you have a better bullet selection to pick from.