It seems to me that you should consult your auto mechanic. I know that sometimes it's very difficult to change the engine in a car. I have an old Toyota pickup, I had some problems with the engine and I decided to completely change it to something more powerful. I calculated the costs and realized that I would leave the engine that I had, just repair it. I often read such portals as https://truckpowerup.com they help me find a solution to my problems with my pickup. Now I have enough jump starter to start the engine if it stalled on the road.
It'll fit as the 1986.5 through 1995 Harbody trucks had the VG30i and VG30E engine options. In 1996 and '97, the V6 was dropped and did not show up again until the 99 Frontier. I've heard two stories as to why this happened. The first story said Nissan couldn't get the emissions certification approved so they just dropped the V6. The other story was that Nissan had a lot of KA24E engines and were trying to get rid of them.
If you are wanting to drop in a VG engine, you can use either the VG30i or VG30E or the later VG33E, which you will likely find it cheaper than the VG30's. There are some differences in the VG33E that you'll have to address, but it will drop right in. That said, it's not just a drop in and bolt up and down the road swap. The 4-cyl. transmission won't bolt up to the V6, so you'll need to get the V6 transmission/transfer case, as well. You'll want to swap out the torsion bars for V6 units. The front end components are also "heavier-duty" than the 4-cyl. Plus you'll need to swap the main harness and ECM and TCM (if auto trans). The diffs are also heavier duty than the 4-cyl. By the time you get done, you'll probably just wish you had purchased a Hardbody 4x4 with the V6 already in it from the factory. I just looked at a 1994 Hardbody SE/KC/V6/4x4 the other day in really nice shape; the owner was asking for $3700. The amount of work and the cost of parts I'm sure would be a lot more than that!