If the plug will start in the hole, only about 1/2 to 1 thread, put a vacuum cap in the hole that just barely fits in it. Best to be too loose than too tight. Shoot a little dab of shaving cream (aerosol) onto the hole onto the cap. Gently run that tap in till the plug will go in about 1/2 to 3/4 way in. Make sure your plug will go in even with the outside of the block with a good forceful hand on the wrench.
Pull your coil wire and crank your engine, or if you don't mind a little mess to clean up, crank and start your motor for about 3 seconds. This will shoot the plug out, along with your shaving cream with all the chips.
Clean up your mess, and get about 3 Q-tips with a little clean WD-40 or New Motor Oil, clean out the hole, making sure you don't see metal grit or dirt. Clean till its gone. Apply a little liquid teflon sealant or teflon tape to your plug. Screw it in.
You can use this also to make your fittings go in further.
Usually the plugs are straight, but if they're tapered, chances are your gonna have to sink then a little. You want to have at least 2/3rds of the fittings threads inside the hole to help hold in in agianst both pressure and vibration.
IF, IF the fitting goes into the hole too deep, and your wondering what to do, go find a ball bearing a little bit bigger than the hole. Put on your safety specs. I have a complete set of punches with a radius ground on the ends for this also. (I save all the old bearings I can find for just such purposes!, or if I need to plug a hole, smack it in, then a touch of Nichol MIG wire or rod seals it all!)
Put you ball-bearing onto a pr. of needlenose plyers and let the ball enter the hole thats too big, hold the pliers firmly, smack the ball-bearing with authority!
What you have just done is 'stake' the top 2 or 3 threads in, reducing the diameter of the threaded hole. Check the fit on the plug or fitting. If its a little tight, you can get on the wrench some, with sealer on it of course, and she'll go into where you need her. If you made the hole too tight agian, barely start the tap back into the hole.....that should be enough for a perfect fit.