Just get hold of a pair of forged jate rings, not the welded ones they may not be as strong, they go striaght on to the chassis mountings and do not mean a load of fabricating or drtilling of chassis members.from £15 a pair with HT bolts.
pop rivets? interesting concept. Would you fancy trusting a bit of steel welded properly into place to return the main structure to its original strength to hold your /your wife/girlfriend/kids seatbelt in place or a few thin hollow squashed out tubes going through some other bits of thin metal...
I assume it is a 3.9, is it using any water at all, if so you may be well worth getting a pressure test done on the cooling system, it may be a leak or a porous or damaged liner in the block.