I'have ARP fitted but I'm only on stage 3 (330bhp) it does the job perfectly.
But for more power it may be smart to go with the long studs. I already saw broken blocs around the treads for the studs. I guess 400bhp is about the limite where both can by used so you will get all and nothing to help your choice!

If your plan is to stay in this stage for a moment fit your ARP kit. if you want more power then you'll replace it with long studs when rebuilding/upgrading the engine.
another good point will be to fit a Cometic head gasket with the ARP or a WRC with the long studs