Hi Rocker,
Yes that is Langdon's adapter.
The '48 had a front center motor mount on the front cross member and a pair of rear mounts on the bell housing and resting on the 2nd cross member. The 4 speed transmission hung off the end with no mounts.
In the 50's the 6cyl changed to the same configuration as the V-8's, two front mounts a few inches further back on the block and a single transmission mount.
I don't actually remember working on a car with 4 motor mounts and a transmission mount. Large diesel trucks, yes. Some straight 8's has 2 front and 2 bell housing and no transmission. I'm sure someone will point out some.
Anyway, Langdon's adapter is held to the block with 6 grade 8 flat head bolts. More than sufficient.
The transmission is held to the adapter by 6 - 3/8" user supplied bolts. Grade 5 would be fine.
The adapter is aluminum and I'm sure Langdon would have used a T-6 grade. 5/8" thick again, more than sufficient for road use, up to the full capacity of the truck. If you were going off road I would upgrade to cast steel.
The bolts holding the transmission to the adapter have 5/8" thread engagement. This a good amount, but if you wanted a belt and suspenders, use grade 8 bolts that are 3/8" longer and put a jam nut on the engine side of the adapter (no washer).
My problem was front and rear center mounts and no side mounts. Nothing to keep the block from falling over and no torque transfer to the frame. This is how most new front wheel drive cars with transverse engines are mounted. That's why most have a top mount between the engine and firewall/frame.
The bracket I added is hard mounted to 2 transmission mounting bolts. The bottom of the bracket bolts to the original bell housing mount relocated higher on the 2nd cross member. This rubber mount provides the isolation.
This transfers the torque to the frame and actually supports a percentage of the engine weight when under load.
The 3rd cross member I added to hold the transmission mount is 1-1/2" x 1-5/8" .125" wall square. The end plates are mounted to the walls of the frame, not to the bottom.
Probably a bit long answer, but almost 30 years as a design engineer, occupational hazard.
