For 270 and 300 grain bullets I have had good success with RL-15, CCI 250 primers and Federal cases. I'm not going to give you a specific load as far as powder weight is concerned since you should determine this by consultation with a loading manual and will vary depending on your gun and the specific components you use.
I start my loads to the middle or lower end of the recommended loads. Generally I've found that as a load is increased accuracy will improve to a point and then the groups will widen again. I use the load with the best accuracy and don't try to go for maximum loads though at times the best accuracy will occur near the recommended maximum. I of course check the primers, watch for sticky cases, etc. in working up a load.