I dodged a bullet last weekend when the screw holding the butterfly to the throttle shaft fell out. Found the screw on top of the #5 intake valve ! Weighing my options to not risk this again. Would you just put it back in with red locktite and keep an eye on it ? Or , since the screw is made of what appears to be stainless and the shaft steel, simply put a small spot of weld on it ? I'm not worried about the need to ever dis-assemble it, but concerned the weld process might warp the shaft. Thoughts ? Is there a better option ?

Dang you were lucky , I would use the locktight myself. I have never been that accurate at that type of welding. So for me it's a no brainer. LOL Locktite and check it every once in a while. My 2 cents

A softer screw and stake or peen the threaded end after torqueing. How the hell did that screw not go through the engine?Lucky.

"How the hell did that screw not go through the engine?" I honestly don't know the answer to that question. As you can see from the pic, the throttle plate is still partially open. I became aware of this when i took my foot off the gas after a 140+ mph pass, shut the motor off at 6900+ rpms. I think I used up all of my engine's 9 lives at 1 time !!!

I agree. A drop of Red loctite. I hardly ever use the green, but that’s just me. Having a stick car, ever bolt and nut has RED on it. Better safe than sorry.

Stake it or silver solder it

Something I have done on bicycle chainring bolts to keep them from loosening up is to wrap a revolution or two of teflon plumbers tape around the threads. It puts them in just enough tension that they won't back out unless you put a wrench on them the whole way until they are apart, and when you do need to take them apart again it's easier to get apart and clean up vs loctite.

Another vote for red Loctite.

I worry about staking the screw without the proper fixture, too easy to screw up the shaft or butterfly.

What I do when re-assembling during a rebuild is to take a pair of pliers or your vise to
deform the threads SLIGHTLY for an interference fit, this has served me well.

My 2 cents, Joe

PS That's the screw threads, not the shaft