What are you going to do to the car that you need to convert to mass air? Unless you're going to be installing a bigger cam, intake and heads there is absolutely no need for it as there is no power to be gained from just a swap alone.

For a 1987 5.0 an entire engine harness from a 1988 California 5.0 or an '89 5.0 is a direct swap being that those years have the MAF sensor harness integrated into them while retaining all the same exact connectors and wire colors for those years (1987-1989; possibly '86, too). Used computers at $100 or more are a ripoff. Remanufactured EEC cost the same and work just as well if not better. I rewired my '88 for mass air back in December of '98 using a Standard™ part # EM073 (A9L or A9P designation unecessary) for $104 with a junk EEC I used as a core exchange at the parts store. Go to the junkyard nearest you and cut out a
MAF sensor harness from a late 80s-early 90s Aerostar, Tempo, Escort, etc and cut out the harness connecting to the EEC along with some length of wiring. Remove pins from junk EEC connector and solder the wires going to the donor MAF harness. I have to look up again in my old paperwork which # pins have to be moved and added. I don't think I spent more than $150 on my swap. It takes patience and time. I scavenged a MAF sensor from an early '90s 5.0 Thunderbird. Works great, although now I'm at a stage where I need to upgrade it.