Welcome! Got any pics? I love the reg cab/short bed trucks from this era, they are sexy! I use to have an 85 D100.
The frames on a lot of the Dodge trucks from that era are all the same (minus the front end between the 2wd and 4wd models) so you shouldn't have any issue bolting up a step side bed to the frame.
As far as the motor goes, you can probably get an intake, headers, distributor, 8mm wires and a larger coil (voltage wise), underdrive pulley kit, depending on the transmission either a shift kit (automatic) or a stronger clutch (center force II), and depending on the rear axle that is in it you can get a llimited slip carrier and swap that in so that both tires spin instead of just one.
Of course you could do what I did with my old 85 D100 and **** it all out for a built 383 BB.