At 78 lbs don't go below 390 grains of arrow weight. It will still be warranteed and it minimizes the risk that you will damage your new bow. We pay alot for these new bows and it's not a good idea ruining them by shooting too light an arrow. Plus I think those .400 spine Goldtip's are way underspined for your set-up.

If you go with the .340 spine you get a total arrow weight of around 400 gr. and a FOC of 10.6%. Plus you would still get over 300 fps with your set-up.