As good as I once Was.
If you were going to set up a work bench with a simple 5/8 plywood on saw horses you would probably not want to put them at the far ends........ but rather 1/3 of the way from each end. Stress as on the six bolts that hold the trans to the engine will be greatly reduced if you get the new mounting brackets. One of the things I like to do is get grade 8 shoulder bolts at the local hardware store for any swaps I do, kind of like added insurance. As for drive line angle, I would worry more about ground clearance, but the 56 we built had a slight slope and the drive line was almost straight in line. I like the idea of bolting the crossmember in! Put a jack under the mount once the other mounts are in place and see if you need a spacer or not. If it all fits nicely drill some holes. I will use hard grade 8 on these four bolts as well and usuall run the bolt with a washer from the inside of the frame down...... then put the cross member and nuts on from the bottom. This will allow you to see it at a glance when under there for service and way more user friendly. I like to spot weld bolt so I don't have to ever use a wrench on the top side, but not manditory. Love that year of car! Oh keep your original parts they are totally worth $ and you can put things back to original if ever need be for resale value.