To be very blunt, the two trucks are not remotely similar. The 1200, which we did not receive here, is also called a Sunny, and is a Datsun 1200 (car) underneath. It's basically a unibody economy car with an open bed where the trunk and rear seat would be.
A 620 is a typical body-on-frame truck. "Common issues" is a relevant question for a modern car. By now, they're all at least 41 years old, so the "issues" would be age and abuse-related. Clean ones are hard to find, as they usually got worked hard and mistreated, although clean examples can be found.
I'm sure you could find some sort of conversion for the front of a 620, but you'd probably spend more than it's worth pursuing cosmetic changes. Lots of engine swaps have been done in a 620 (I have an SR20 in mine) but keep in mind, most are a kingpin front suspension and the steering box / arms / crossmember limit modern engine swaps. Still, people have put damn near everything under the hood - it just depends on your level of fabrication skill.
You'll see pics of 620s slammed to the ground all over IG and FB. They're primarily for show, or have had significant chassis mods. The front crossmember is very low, even at stock ride height, so unless you're bagging it and dropping it for pictures for kids, it's not terribly functional for driving.
We have a huge section here (and on DatsunForum.com) full of Datsun truck folks, so do some reading and soak up as much info as you can.