Are you planning to add some a web to your cross-member? 1/2" plate doesn't have enough section to support half your drivetrain.
X3, I also used 1/2'' plate for my skid/trans mount... But mine is about 3' wide, not 4" and I still plan on adding bracing when I pull the tranny out to replace it...
I can jack the whole truck up on it and it will balance when you get it right, but it does deflect a little bit...
If you don't brace it and come down on that crossmember, it WILL bend...
And not to bust your balls, but I wouldn't trust a 110v mig for motor mounts or suspension links... I wouldn't even trust a 110v if I were running gas... 110 is nice for tacking or sheet metal, but you need a 220v for anything beyond 1/8" IMHO...
If it were me and I didn't wanna go get a 220 mig, I'd go to Home Depot and pick up a 220v Lincoln "buzz box" (arc welder)... It'll give you plenty of penetration and you won't have to worry about the welds breaking. Plus they're only like $200..
Just get some 6013 rods (easiest to learn on and a good all around rod) practice for a few hours and you'll be good... Plus you'll have the satisfaction of knowing how to stick weld (which is a good skill to have.
Ok rant over... The trucks looking sick by the way!