I think the part you're asking about is the bumper bracket. They're available from anyplace that sells Model A parts.
The bumper and it's hardware should be the last thing you need to buy.
What the guys said about the chassis is true. The Model A chassis is probably full of stress cracks and rust already from years of use. It's not strong enough for anything more than the original drivetrain.
If you're a good welder you can box the frame. Boxing the frame is a lot of work. Making your own frame from 2X3 rectangular tubing is less work and you get a stronger frame.
AL