I just thought of something, you know the 83+ regular cab is bigger(3 inches???), and the box is that much shorter, if you could find the larger regular cab and box, it will likely bolt right on to your frame, but anyone that knows their shit about Datsuns, will know it's not a 1980 cab/box.
In the end, it's just easier to buy what you need already made(stock), take my word for it, it's never just more legroom.
I have found that the bench seats have the most room in regular cabs, as they have skinny backs, another thing you can do is raise the bucket seats, the higher they are, the more legroom you will have.

“The difference between genius and stupidly is that genius has its limits” Albert Einstein