Summit Racing offers the two piece mounts bolt in.
The crossmember is offered in The JC Whitley Catalogue. Its way past the lighted hood ornaments, curb feelers and Night Rider grille lights. In the last book I had it was towards the end. " Universal cross member mount, allows the installation of small block Ford and Chevy engines in many earlier frames 1928-64 Ford truck"
This mount is offered by many suppliers.
In 1991 I installed this mount in a 54 GMC. This is what I did.
The front clip was off the GMC so I had plenty of space. The engine in the truck was mounted hap hazzardly. The the old style front mounts were not holding the engine a SB#% well. When it was revved the engine would twist so much that the valve cover would hit the firewall. A small length of chain was wrapped around the exhaust manifold and frame to help keep it between the rails.
I inspected my new mounts. I also ordered the engine pads and mounting bolts.
It comes in three pieces. The two frame pieces are made of a larger diameter tube welded to a flat square plate. These fit against the inside of the frame rail. It has four holes drilled for mounting to the frame.
The cross member itself is of slightly smaller tubing it fits inside the frame pieces. I took measurements of the distance between rails. This is an eyeball fab job and constantly measuring was not nessesary.
I cut my cross member an inch or so shorter than the inside rail distance. This allowed for the flat plate that bolts to the frame.
I rigged the engine like I was going to pull it. I had two eyes bolted across the intake this is where I had my chain bolted to alow me to lift the engine with a hoist, come alongs back then. I also chocked underneath with blocks of wood. I then proceeded to remove all that jerry rigged P/O crap.
I put the three pieces of the cross member together. I placed one side in the rail then swung the other into the other rail. With it in the rails you can slide it up and down the frame under the engine. The GMC had no fuel or brake lines in the rails that got in the way. If it had they would need to moved out the way until the job was complete. With the mount centered under the engine I bolted the pads on the block. I then made sure the engine was centered and everything lined up. I even took measurements from the front of the frame to the cross meber to make sure everything was square. This was a test fit.
With chalk I then marked through the bolt holes onto the inside frame rails. With the engine still secure at several points I slid the assembly out of the way and drilled my holes. I used a small bit first then bigger. Slid the assembly back into place and bolted it down with 8 bolts.
The cross member would swing inside its mounts but it was a physical impossibility for it to fall out.
I then mounted the motor. As the engine was lined up and set down the weight locked the cross member in place. As long as the transmission is mounted its not going to move.
Welding the cross member tube to the frame mount tube is a good idea. Honestly I never had that done to the GMC. You couldn't move that motor in its mounts with a 6' pry bar I tried. I would get it welded if I were to do it today.