those lower threads on the aluminum timing cover are a weak 11/16" deep & open on the back side & go directly into the timing chain area/eng oil & it uses a 3/8 bolt so I'm thinking if you could block the very back with something fairly firm just to temporarily plug the rear end of that passage for your work then drill/tap a good amount of the threads then carefully/lightly blow the chips out the front (the critical part) & I would use brake kleen as opposed to shop air & either helicoil or just drill/metric tap to the next larger metric size/use a metric bolt. Use grease on the tap to catch alot of debris. EDIT Another option might be to permanently JB weld a stud in there