It really depends on what you want to do with the truck, fabricating experience, tools, time, and money.
1975 was the first year the 620 had the L20B, which is a good all around motor, dont like to rev very high but it has good torque.
Since you brought up motor swap, lets talk about that some. There are motors that will bolt in directly without much work.
The last year of the 620 was 1979, 1980 was the 720 model but used the L20B for the last time. Then they went to a Z22 and Z24 engine, which is bigger.
The later Nissan hardbody trucks came with a KA24E engine and also bolts into the same spot, with a few changes.
Those are if you want to stick with Nissan bolt in parts, otherwise you could always drop in a 5.0 or chevy V8, requires alot of drivetrain fabrication.
I had a 74 640 with a 86 Mustang 5.0HO motor, C4 auto, 8.8 rear end swap, and custom driveline with adapted brake lines.
Worked great, did a 12sec quarter mile.