Aaaaaaaaghhhhhh!!!
I don't think that's a hand drill job. I'd want the block out of the car and taken to a machine shop for HeliCoil installation. You want those holes EXACTLY perpendicular to the deck surface. All kinds of bad things can happen if the head bolts are not right on the money. And, whoever you have put them in better know what they're doing. You don't want a bolt pulling out.
FYI: Virtually all fasteners on these cars (and most all cars since the 80's) are METRIC. Man, that's a tough lesson to learn the hard way...
JD