The reason they say that is with a bigger,longer duration cam,your bleeding off cylinder pressure do to valve overlap,it'll sound wicked and be a total dog unless everything else is matched to it.. the best running basically stock 350 I've ever had,I put a comp. cam 268 in it..around .454 lift I believe.. unless your willing to up the compression,add headers and good exhaust,change gearing and convertor,keep the cam size relatively small.you'll be much happier with it's performance..
Tim