Sort of a coincidence.
I've just finished putting ARP studs in my TR4A block. I could not "hand tighten" four of the ten studs, including both 9.5 inch studs, so I called ARP and Moss Tech Support.
The consensus was that there must have been grit or debris on the bottom of the seats. I swabbed them with a homemade engine Q-Tip dipped in oil, used a gun barrel rod and wire brush attachment to ream them, and then dried them out. Then blew out the holes with compressed air.
My fear was stipping the threads. In the end, on three of the four I had to apply a little force with the allen wrench fitting on the top of the bolts, and all seemed to seat well enough.
Are you sure you didn't just strip the last few threads; because there's plenty of thread below those. On my car the final adjustment had each stud sitting about 13/16 above the head face.
If you did badly stip the threads, I'm told a heli-coil will do fine, though if it were me I'd need a shop to handle that.
I'd like to hear how it all works out. Good luck.