Listen up, time for Heat Treat in your shop entry-101. Steel is a crystalline structure. The crystals are changed in size when you heat and cool the metal. Slow cooling, (big crystals) equals soft metal. Fast cooling, (little crystals) means hard metal. You have to heat the metal to about 1850'F to make the crystals all loose and runny, ready for the quenching process. You cannot do this with a rosebud torch tip. You have to use some kind of furnace or kiln. The metal has to be a bright orange. The whole thing, not just in places. If you put the sheet metal into the kiln when it's cold, and let it heat up slowly, you will experience the wonder of oxide scale. Basically your shiny receiver will turn into a piece of weld slag. The kiln must be hot and the receiver must be placed in it for no longer than 5 minutes max.
When you take it out of the kiln and walk across the shop to place it in a water quench, you lost it. The metal will cool below that little crystal stage and go into the big crystal stage before you take two steps. The thin sheet metal isn't "massy" enough to hold it's heat. You have 1 1/2 seconds max, to get it into the water before it turns out dead soft. You will know if you did it right if it has a nice "ting" when you tap it. If it goes "clunk", you loose. The up side is you can try again. You can do the process over, several times in fact. Metal is not damaged by the heat treat cycle.
The trunion could be salvaged by the same process, but it must be tempered. Tempering involves re-heating the metal to a medium temperature for some period of time, called a "soak" and the slowly cooled to room temp. This take away the over hard brittleness and makes the metal tough. Problem is, who knows how long and how much? Each alloy has exact reheat and soak formulas. These are based also on the thickness and mass of the metal part. We don't know exactly what it's made of or the time required. So you're kind of lost here.
I have made receivers from scratch, so you're getting the facts here. 4130, the sheet metal alloy, is more resistant to hardening than 4140. 4140 may "over harden" and need a soak at around 800'f for 10 minutes or so. This temp level can be checked with a heat crayon available at a welding supply shop. The wax will melt and run when it reaches the temperature you want.
Two presses are just about required to build AK's. A drill press and a hydraulic press. Both are useful for many things besides AK's. Both are worth investing in.