The easiest way is to use a gas shielded mig and clamp a thick copper plate to the back side then weld the other side shut, remove the copper, weld a bit on the back then grind.
It is also easy and nice looking to tig weld them.
You can also stick weld with the copper piece behind, that works good to, but better when it is vertical so the slag runs off instead of puddling up in the hole. You can weld some, stop and remove slag and weld some more though.
Hope this helps,
Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-