Patience!

This plate is too small to deal with restraightening. Just tack the pieces in place, then jump/scatter weld it 4 times, go get a beer and drink it slowly, then come back and weld 4 more times, then go get a glass of water, then weld 4 more times, and get another beer......

And 3/8" welds aren't really welds, they are big tacks and prone to breaking if these bars are really necessary.

Also make sure you weld on both sides of the flat bar to keep that straight as well.