The big thing when making that crossmember is driveline angle. Get yourself 2 spirit levels, and an angle finder. Since you will likely have to make the crossmember with the new transmission in place as a jig of sorts, you will need to do a few things.
Firstly the car needs to be dead level front to back. Figure out how high up in jacks you want/need it to be, then level it front to back using spirit levels "torpedo level" on each of the door sills. Remove the carpet sill plates and mount each level to the door sill itself. Now shim the jack stands with steel sheetmetal stock until both levels are level. Put the jack stands on the frame rails, not the suspension.
Secondly before removing the old transmission, remove the carburetor and put the angle finder right on the carb mating flange on the intake manifold and note the degrees of down angle of the drivetrain.
Thirdly, with the new transmission bolted up to the engine, you have to maintain the original down angle. Whatever angle that angle finder shows on that carb base on the intake manifold is what you should strive for when building your new mount.
You can build it where it let's the trans droop maybe 1° lower, because you can always add a shim or 2 under the rubber trans mount to bring it up a degree if you have to.
Stick a jackstand under your new transmission and shim it to get it to where the angle finder read before removal of your old transmission.
At this point build your new mount to fit the chassis, and transmission using the car and transmission as your jig.
