I plan on closing off my front side marker light holes before body work & paint.
I have only very minor work elsewhere, so the signal hole closeout will be the hardest of all the body work.
I can weld & have tig, mig, and even oxy-acetylene if I got some bottles.

What welding techniques worked for those that did it?
Full weld, tack weld only, seam weld, braze, filler, no-filler, copper backing plate?

Any experiences out there?
Both sides look like swiss cheese because the PO took off the lights and pop riveted covers over the holes drilling ~6 extra holes in the process.
I am worried that too much heat will warp the fenders which are otherwise perfect.

I hear that dollying/peening the area gently right after welding relieves stress, but isnt that also how you enlarge steel to "stretch it" too?
Doesnt Heat & dolly = bigger?

If there's that many holes, cut a larger section out and make a bigger patch. You'll end up with less heat. Just work slowly and you won't warp anything. If you can tig, use that. If you mig then leave a bit of a gap between your patch and the fender metal. HAmmer/dollying does stretch the metal, but welding shrinks the metal, so you're counteracting the shrink.

How the worm turns. The introduction of side markers by federal mandate drastically reduced what are commonly called t-bone crashes at night. In your case without the markers, you would be at the top part of the T. I personally, using my own flawed risk calculator, feel this is dangerous. Backed by those statistics and a bit short of 50 years driving for experience chops, without an accident. If the car is for shows, all bets are off. If you think it will only be out during the day that helps, but is no guarantee. And as mentioned before any modification of such safety devices would potentially put you in the wrong in court.
I hope we are still buds.
Ok. First cut a patch for the big hole and gently butt weld it in. You may need to do a tiny amount of hammer and dolly work after you grind down the weld. In general, if the hammer doesn't ring against the dolly you are shrinking and if it makes a clear ringing contact the metal is stretching, but that is awful general. For the little holes I would simply close them with little buzzes from the mig without reducing amps so you get good melting. You know it's too hot if the hole gets larger instead of smaller. Copper backing helps but also changes penetration. Actually, I doubt it will be a problem, although am sorry about the extra holes. Then you can lead or bondo. I recently started lead work on a 911, but it is much harder on a vertical and frankly I doubt it is better. Use your hand instead of your eyes for body work. You feel flaws that are invisible. I hope this works out. I do think you are safer with the markers, but really it's your call. Onward amigo!

Cut a patch the size of the hole and tack weld (stitch weld) the seam starting at 12 then 6, 9, 3, and keep going around the circle until the seam is filled. Tack weld the 6 holes shut and grind smooth.

I'm kind of with Worn on the need for side markers, but I think the small square 911 turbo side markers, rear of the wheel look better than the round warts up front.

I'd opt to make one patch for all three holes, since it would be easier to weld.

Round or curved patches work better than squares.

His car is a race car. No need for side markers.

Tig them if possible. Less heat and chance for distortion. Give the welds a chance to cool as you work your way around. A copper backer/ welding spoon is very handy and helpful. You'll be fine.

Hmmm, I have to admit there is little safety data about racing with or without those side markers. I stand corrected. Stu - carbon the whole fender. I just saw a post about that not long ago.

Tough to hold a backer when using tig.....kinda a bitch, I tried .....

OP, just cut out holes, make plugs that match by tracing from front while holding.the piece up from the back.
I flush mount the patch and seam weld it, slowly....give it time to cool so you don't warp the panel.
Grind it, hammer and dolly if it's not flat, sand it up to 80 grit...off to the body shop.

If this is a wheel to wheel race car, don't get to worried about the front corners, they may not look great after a few seasons anyways with all the crap thrown up at a track...

Sorry I missed the race car part. In that case, just put a piece of metal duct tape over them

I went with Vette adhesive in a fiberglass matrix from the rear, then a thin coat of filler on the outside. Sanded, painted, done. Oh, don't wet sand filler, most of them soak up the water and swell. A few months after painting you will see the original holes as little dimples... (I figure the water had nowhere else to go but to attack the steel, so I'm expecting the whole patch the become a gaping hole someday!)