I just did this to my project tractor the other day. I was using 3/8-16 x 1-1/4 grade 5. I bought this engine from someone else (who assured me the mounting holes in the aluminum oil pan were good) and stripped out the threads in two of the four holes without over torquing the bolts. I bought a cast iron pan off ebay for $40 and replaced the pan. I was going to look at a heli-coil repair, and may do that if I ever need that pan again. I think I'd go ahead and do all four holes. You want to continue to use the 3/8-16 bolts to hold the engine when you're done.
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