Basically, the FMU needs to be put downstream of the fuel pressure regulator. The 96+ 420a's have the regulator at the back fo the car by the fuel tank, so, the FMU need to be put in line with the return line to raise line pressure.
The 95 Fuel systems ahev the regulator directly on the fuel rail, so the FMU is located in the engine bay.
If you have a 96+ 420a, and want to put the FMU in the engine bay, that will require either moving the regulator to the engine bay and routign it from the stock regulator to the new FMU.
Anotehr option is to replace the Stock regulator with an adjustable FMU so you can set your own idle fuel pressure.
I woul deither get the Vortech fully adjustable FMU or the Bell Engineering Adj FMU if you are interested in that.

Vortech FMU
http://www.vortechsuperchargers.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=VEOS&Product_Code=6X100-001&Category_Code=FC

Bell FMU
http://www.bellengineering.net/miataextras.htm


Wyatt Leras
http://www.turbogs.dsmpower.com
98 Eclipse GS Star Stage 5,000 Turbo kit