I just went to look at the plug going to my "regular" TPS - if you want to save it without cutting you may be able to use the smallest blade type connectors. I'm not talking about a OEM style plug but individual crimp type connectors on one wire per connection ( 3 total )
There is one blade type male connector sold at auto stores and maybe even RS that is about 1/8 wide. You would need to cut them down with a pair of sharp wire cutters and then file them smooth and flat.
Once trimmed you should be able to insert them into the factory harness that plugs into the TPS. The wires coming from these connectors will go to your Variable TPS
Even with the OEM '90-'93 Auto TPS, it's not a plug-n-play mechanical matchup for the MT TB. On the manual throttle bodies there's a long shaft which engages a hole in the throttle switch. On the auto bodies, the shaft is short and has a flat plate with tabs protruding from it, and these tabs engage the TPS.
When I converted mine, I cut down the shaft on the throtte body, tapped it, and then used the lathe to fabricate both a small, mushroom shaped piece to sit on the end, with a small stud sticking out to engage the TPS. I then lathed a collar to center the TPS on the throttle body and space it properly. (I used a Honda TPS, but it'd be basically the same gig for a Miata TPS.)
Long story short- no matter which TPS you buy, there's going to be some fab work involved. So if you've got a line on a cheap new Ford part, might as well get it and make it work.
I went through duralasts catalog to find the TPS I used, I really wish I could find the part number, because the fab work took me about 15 minutes. IT consisted of marking the postion, removing the guts, screwing two screws and soldering three wires.
really it's not much different than Ben's write-up, i just didnt have to cut the shaft or cover.
The threads probably cross polinate but maybe it will help someone if they are all in one place .
I remember reading someplace tho, that the VTPS from an auto does not bolt on to a 5 speed TB because the TBs are different between the two cars ( auto and 5 speed ) That seems to be what Joe describes in the previous entry.
When I pick up my evil spawn from school I'll be able to tear off the TPS and see what it takes to convert.