I know that what follows is not what you wanted to hear...
I get that you really want to make this out of a classic Datsun so it fits your needs and your desire for looks, and although I do agree with Lockleaf about the Brat being a good fit, they are somewhat hard to find from what I've been told by Subaru friends, so the long and short of it, if you are dead set on a modified vehicle it would make way more sense to truck a Civic to fulfill your needs than to heavily modify a 620 to fit.
The plans you have for modifying a 620 require extensive fabrication skills and a lot, I mean a lot, of money. Plus it sounds like you have a timeline where you want this sooner rather than later. That said, I really think starting with a Civic and "trucking" it would in the end be a better choice to fits your needs.
It would already have the FWD platform, already get good MPG's, already be a short wheelbase, once "trucked" it would have the shortened bed you're after, and all that is required is the much shorter and easier to complete fabrication of the new rear wall.
There were even a few pics out there of clean looking conversions, but of course those pics are hosted on Photobucket so I can't link them here. Google civic truck and you'll find them.
Feel free to completely ignore my post and do whatever you want, just thought I should throw in my 2 cents lol
Oh, and the obvious easy button, but higher buy in, is a VW Caddy with the diesel.