As KCM said.......I have used a small drill bit to make a pilot hole straight through the centers of the spot welds then come back with a large bit about the diameter of the spot weld to cut through the top layer. Has worked for me on several of my classic cars in the past. Then drill holes in the new part
about the same factory spacing as the old welds and weld through the holes with a Mig Welder to form a plug. Grind them off with an angle grinder to make them look like a true spot weld.

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K