Hey BJ,
if you have a drill press, drill thru a 1/2" block of steel then clamp it over the boss as a pilot hole. Useing a 5/16" drill/3/8" tap drill. I think you can safely drill 5/8" deep after it's drilled open up the guide plate to fit the tap and bottom tap with a 3/8 X 16 tap, then put a stud in the tapped hole, this will keep everything square.
If you have a 90 degree drill it will be a lot easier if the motor is still in the car.
A couple yrs. ago we installed a 400" blk in a dragster, only to find afterward it wasn't drilled for straight across starter bolts. I made a guide plate grabbed a 16" long drill from the shop and had it bolted up in 1/2 hour laying on my back on the garage floor.
JMO
Zip.