When I need to drill /tap out anything that is a primary mount hole or for something else to bolt to a engine(other things also) I make a simple guide piece out of whatever I have that is fairly thick-In this case I would start off with drilling 3 holes in the piece that were TAP drill size------holes would be like A,B,C---A would be for the location of the present already tapped hole in the block and B and C for the other hole-then drill A out to the mount bolt size (close tolerance drill size) and either B or C will be used to guide the tap drill into the block SQUARELY( use a stop collar to limit depth of hole)--then use the remaining hole either as a guide hole using the tap threaded thru it also or as I would have it as guide for the tap at tap od --------- This will require moving the guide piece to the second hole or if you want a one time thingie you can do it with only two holes------
I also on pieces I need to use many times will buy and use drill bushings-----