I have seen several solutions, and i figured out a simple one for my liking, if you have access to a welder.
I used grade 8 nuts. These nuts are spot welded to the frame anyway, and the frame is what supports the engine weight. the previous owner had already drilled the holes for me, I just figured how to spot weld a nut in there, so when you tighten it, the bolt wont move. The bolt does not hold any weight.
so I welded two nuts, to a flange...

make sure your bolt is center and is appropiate

I secured the flange with the bolt into the right position, to center it and to get it tight to the frame, where it would rest forever...

once welded properly, i cut the excess metal..

did the second one and filled up the holes again, to prevent mice from finding a nest..

and shaved, and had the bolts ready to be used..

those nuts dont move and you can torque the bolts appropiately... I was in a hurry, so i could have welded the holes.... but oh well.
This is not the best approach, but just one of many...
I used grade 8 nuts. These nuts are spot welded to the frame anyway, and the frame is what supports the engine weight. the previous owner had already drilled the holes for me, I just figured how to spot weld a nut in there, so when you tighten it, the bolt wont move. The bolt does not hold any weight.
so I welded two nuts, to a flange...

make sure your bolt is center and is appropiate

I secured the flange with the bolt into the right position, to center it and to get it tight to the frame, where it would rest forever...

once welded properly, i cut the excess metal..

did the second one and filled up the holes again, to prevent mice from finding a nest..

and shaved, and had the bolts ready to be used..

those nuts dont move and you can torque the bolts appropiately... I was in a hurry, so i could have welded the holes.... but oh well.
This is not the best approach, but just one of many...
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