I struggled with that cap screw myself. With a 1/2" breaker bar (18") and all of my strength, I could not break it loose while working from the back of the transmission, using extensions. I used 1/2" extensions with an adaptor for the 3/8" bit on the end.

So, I got a piece of pipe to fit over the breaker bar that was about 24" length and there was just enough angle to get the contraption up in there and torqued enough to crack the bolt loose. When it let go I though I was a goner. There was a crack that sounded like something fractured. But, the bolt had budged and from then on it was downhill.

You need a 24" stiff pipe (1.5" diameter is nice) and just about all the strength you have. While doing this, you must make absolutely sure that the socket bit does not slip out of the socket head on the cap screw. I can't recall, but I probably used the universal in the set of extensions to be sure that the bit went squarely into the screw.

I'm not a fan of the impact wrench. Too much localized force on the inside of the screw. The chances of rounding it off are very high. You don't even want to think about the consequences of this.

It's a difficult and miserable job. You have my sympathy. In my case, life was much easier after the screws were removed because I did the job with the manifolds off the right side of the engine.