Red, I'm not familiar enough with the 1984 F150 to know if there's room under there for a second muffler. If that's what you're after.
If all your after is the duel look, any competent muffler shop should be able to do that for you.
If that's the case almost all the after-market companies, Flowmaster, etc make single in, duel out or duel in, duel out mufflers. Whatever you need.
I originally had a Flowmaster 40 series single in, duel out put on my 3.0L Ranger. Had a number of folks tell me they thought it sounded like I had a V8 under the hood.
I didn't think it did, but that's what they said. It was much to loud for me (but then I'm 69) and the in cab resonance drove me crazy.
So I took off the Flowmaster and replaced it with a Magnaflow. I had a muffler shop bend the pipes for me so that I now have four (4) exhaust tips out the back.
Check around with a few shops in your area. It shouldn't cost that much. Or if you have access to a pipe bending machine you might give it a try yourself.
You should be able to find the mufflers on Ebay for about $70-80.

Here's what the back of my truck looks like. Click on the photo to "super-size" it.