On to the actual mounting of the engine and trans. I'm sure somebody makes a mount that will drop the motor and trans into place. I will take the contrarian point of view and suggest some ways to do it.
Here's a crossmember that accepts stock-type Chevy motor mounts and bolts or welds to the frame....
http://www.speedwaymotors.com/S-B-B-...ount,3273.html
Uses these rubber mounts...
http://www.speedwaymotors.com/Small-...unts,7959.html
Or these polyurethane mounts...These may transfer a little more of the vibration of the motor to the frame/seat/your caboose/steering wheel.
http://www.speedwaymotors.com/Polyur...DisplayId=7959
Here's another mount system that may work with some tweaking. I have used this arrangement on a few cars. Works out sweet.
http://www.speedwaymotors.com/Chevy-...unts,8473.html
Here are the instructions....
http://static.speedwaymotors.com/pdf/720-4771.pdf
Here are some stands that will work with the stock rubber motor mount. Just trim/cut the stands for the location where you want to weld them in....
http://www.speedwaymotors.com/Univer...unts,1905.html
Maybe this arrangement would work for you. These are AD Chevy frame adapters....
http://www.speedwaymotors.com/1949-1...ters,6673.html
That work with this mount....
http://www.speedwaymotors.com/1955-1...DisplayId=6673
Here's the install illustration....
http://static.speedwaymotors.com/ima...neMounting.gif
And instructions....
http://static.speedwaymotors.com/pdf/135-6023.pdf