I am also driving a 91 it's a Nissan hardbody 2wd 2.4L with 330k on the original engine. That's why I have always driven japonese and also where I got the name JapScrap. I'm pretty sure your truck has a 2.6L so our trucks should be fairly similar in power, your 0.2L should make up for your extra weight. SHOULD. Anyways I, like many have suggested already, bought a trailer. It's an all steel 4x8 with 4' sides and only cost me 300$. Sure I did have to fix it a few times but I had saved appropriate pieces of steel ahead of time from my scrap. The most I have ever hauled in the trailer and truck combined was about 4000lbs of industrial racking. It was scary as h*ll for me and my clutch and though we both survived I would never do it again or ever suggest anyone try.



Point being you can probably comfortably tow about 2000-3000lbs.
Oh and no one else mentioned it yet so I will and I speak from experience... Gat a hitch if you don't have one. At first I towed with my bumper (rated at 2000lbs) I never towed more than 1500 incl. the trailer. I found out pretty quick that after 22yrs my bumper couldn't take it and started cracking in four different places. Picked up a brand new curt 5000lb hitch for 225$. It can handle more weight than my motor ever could.
Best of luck!
Oh and another tip if your trucks bed is a 6 footer then the two halves of a steel futon should make the perfect size sides. If you drill holes in the bed rails at the front and back then stick long pieces of rebar in the two holes and then you can just slip the futon halves over the rebar and voila! As for the window I built my own "back-rack" with 1" square steel tubing it took about 1/2 hr, but you could always use another futon half. Cut however much you need out of the middle then piece it back together with steel tubing that will fit inside the horizontal bars. You can use plumber's strapping to hold your 3 sides together. You don't need to secure anything to the truck itself as gravity will keep it all in place,if your rebar is long and thick enough, and you can just take it all off when you want.
Sorry for the rant. Hope it was with reading and let me know if you need any more info or pics. I'd be happy to help!