I made my frame wrench inserts in a manner similar to you. My wrench itself is made from angle iron, 2 pieces that clamp together with 1/4 20 bolts and hold the inserts tight to the frame. One piece of angle has a piece of round stock welded to it for a handle. Its ugly but works well. I use oak blocks to clamp my barrels.

I have made a ejector rod lug using a piece of square stock, drilling it for the spring and lug, then using a end mill to make a slot for it in the bottom of the barrel and silver soldering it in place.

I have done a few stainless barrels with no problem. They are some type of 400 series stainless and I don't think it galls like 300 series can. But, I understand your concern. Once stainless starts to gall it goes south fast.