Hi I'm a straight six guy myself ,I would find a good running Chevy 292 straight six hooked a NV4500 5 speed to it ,then adapt TBI fuel injection and enjoy the truck .
This place below even has the wire print out online for the TBI computer if you want to do it yourself .If you really want a auto trans ,I've seen that the th400 can be used without a computer with this special electronic box you can buy.Remember heat is the auto trans killer use a good trans cooler .As for motor mounts most later Chevy trucks you can unbolt the motor mount perches .
You would measure all out with the motor and trans hanging on the engine hoist ,once you get it right place build your trans mount first and get it bolted in place , then bolt the motor mount perches to the motor mounts on the engine then build your engine cross member with 2"x2"x 1/4" square tubing .Pie slice cut the tubing without cutting through the tubing so will have one continuous piece then 3"x3"x1/4" angle on each side that would sit on top of the frame rails .
Better yet take a look at my drawing I made up .Just PM me if you have any questions .
I have a 250 six and NV 4500 5 speed to put in my 1948 2 ton flat bed .Still have to collect a few more parts before I do my swap .
Hope this helps paul
Drawing here with some pics also(sorry you'll have to copy this link to browser)
http://www.flickr.com/photos/58989402@N08/sets/72157651706555116/
Hamilton Fuel Injection
http://www.hamiltonfuelinjection.com/
