There were two types. Both mounted a little differently but looked the same when it was all done.
The early type was a 1947/48 KF radio installed with a kit -
I have the instructions for installing this type and I believe pnw_oldmags has them on his site. It was supported by a simple "L" shaped metal support which was bolted to the firewall.
Here's the diagram that came with the kit:
The other was made specifically for the Henry J but I've also seen this type installed in Darrins. It's this one -
This type has two support struts that mounted through the cowl (top of firewall) and most Henry Js have dimples there so you can easily drill the holes to mount this type. AZ may have the instructions.