Mike the location of the steering box is pretty important. If the IFS is what you indicate, the steering box should be forward of the IFS a bit and spaced out from the frame. From my research the most common problem in making this particular swap successful is getting the steering box in correct relationship to the IFS.
You are likely to need to use some u-joints to get between the steering column and the steering box. But those issues are not as difficult as getting that box in the right place.
Regarding engine compartment length, mounting the motor ahead of the firewall isn't hard. Getting a big enough radiator in place ahead of a big block motor might be a problem. I've seen it done, but not owning an Art Deco truck I haven't paid attention to the particulars.
I can't help you at all on the non-GM modern motors. I wouldn't be inclined to even try any of them. The more modern GM motors mount a little differently than the classic V-8's, and they are a little more complicated to set up because of the EFI, but there is no reason you can not run one in a truck like you describe.
