A little trick I have used before if you can get the bolts started a little bit is to get another bolt of the correct diameter only longer. Cut two v- grooves in the end of the bolt two or three diameters long. Then take and screw the bolt in until it gets tight working it back and forth. When it gets too tight bring it back out and clean the grooves. You may have to run it in and out several times but this will work, just don't force it, go a little at a time. This will save the expense of a tap.
Fred