I did this on my 59 GMC 270 with good results.
With pan off, tapped with 3/8-24 unf, no drilling.
Did this with everything bolted to the crankshaft as is.. afterwards i cleaned with brake/carb cleaner and sucked up the chips with a vacuum cleaner..
Just put a dash on gasket material on the new bolts so they wont leak, workes for gmc 270, i do not know if the hole sizes are different on Chevy's.
