i agree with mbdoc this type of repair needs to be square and straight. in drilling a hole by hand you will get out of round holes, crooked holes, oversize holes and all sorts of pure ugly.
now if you were on a cast iron block you could use a mag drill but.....
as for the tapping you need tap holder to start the tap straight. it's kind of a tower that holds the tap&handle perpendicular to the surface being tapped. also don't use a 4 flute tap and don't buy a hanson tap from ACE hdwre. they are for chasing threads not really for tapping because they are not High Speed Steel and a 4 flute tap is nothing more than a hole plugger).
what you need is the right equipment but most DIY don't have all the right stuff. you didn't say directly but it appears that some of the previous kits weren't installed correctly.
i'd let a wrench do this one. he has the equipment and special tools needed for the job.
if you do, decide to do this job a little trick is to use a can of foamy shave creme. after you pack off the block set up a catch area by taping up a wall around the hole you're working on and fill it with foam shave creme. the wall will keep you from throwing any shavings that the foam doesn't get.
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