Weld, it's about time.

With the frame piece machined, it's time to weld it all together.  Almost.  As you can see, the angles of the two pieces didn't match and there was a lot of overlap.  So I had to fixture and mill them to match.

To hold everything while I welded, I made myself a jig.  It's a pretty basic jig that just locates on some pin holes.  Because the exact alloy is unknown but similar to 4140, that's how I decided to weld it.  Preheated it up to around 400°F and went for it, then slow cooled it back to room temp.  All the impact from the slide and barrel are on the new front section, so weld strength isn't something I'm overly worried about.  Plus, they made these frames out of aluminum too, and even soft 4140 is tougher than 7075 aluminum.

With the welding done, I could get all the parts fitted and functional.  I haven't done any cosmetic finishing on the welds yet, just did what I needed to make it work.  For what it's worth, my P38 parts kit was made in 1961, though I bought some of the early style grips for it, which are the correct style for Megatron.