It's not so much the cleaning of the holes, or the tapping of the threads, but the SHOULDER problem, the ARP studs have a shoulder at the top of the block thread end, this shoulder absolutely 100% MUST butt against the blocks recess.
It seems some ARP, or rather some blocks have slightly different depth holes, so really careful looking at the shoulder needs to be done to ensure it is butted right up, doesn't need to, and shouldn't be done silly tight, but a few mm may be needed to be taken off one or two.
Of course as said above the holes needs cleaning out of old congealed oil, the threads should have a tap ran down them, with greased tap to pick up the rust etc in the threads, then depth check the holes, it's a totally different feel if they stop due to the shoulder or the studs bottoming out.
Then just fit studs finger tight, then when they butt in the recess around each stud hole, a pair of pliers on the shank and give a twist, don't go mental, 1 hand on the pliers, use your weakest hand, it will be more than enough.
tabetha