When you use a head bolt and torque it down, due to the design of the head bolt it has to be rotated in order to engage the threads and secure it, doing this creates a twisting force and a clamping force which means that when the cylinders begin accumulating the load, the bolt will stretch and twist. Basically because the bolt has to react to two different forces simultaneously, its capacity to secure the head is slightly reduced and forms a less reliable seal in higher powered engines.
A head stud however can be tightened into place without any direct clamping force applied through the tightening process. The nut provides the clamping force rather than the torque of the fastner itslef, therefore the rotational force is avoided entirely.
In my opinion while the engine is apart buy the ARP head studs and also the mains, go for the ARP 2000 as they are very good. If in future you have to remove the head again, you will not have to replace the ARP studs and can re-use them.
For the sake of £150 odd upgrade to the studs.. I'm not sure on the modification required to one of the mains as you have stated perhaps calling a reputable RB engine builder will help you.