If you already have this dually I want you to check and see if the connection to the drive shaft, pinion flange, is the same as yours. I have heard about adapters but can't find any for trucks that early. If the dually axle matches then most other Toyota axle will fit too. An 86 and up will give you and extra 3 1/2 inches of width. 2wd if you want all your wheels to match.
A Tacoma axle will give you 60 inches of width but you'll have to upgrade your front wheels to 15 inches and the gear ratios aren't optimal. Seems all the 5 lugs came with 3.58 gearing. Most of ours are 4.11 or 4.10.
I'm pretty sure I recommended single wheels with your existing axle. Your rig is light enough and it would be the cheapest. I've never changed out an axle so hopefully someone who has will comment. Fred switched his 78 to a GM axle so I'm sure he knows more about getting the drive shaft connected