To minimize problems that can leave you stranded, mount it vertically with the connector on the bottom for drainage. Do not seal so moisture can evaporate. Use dielectric grease liberally in the connections. The board should be encapsulated but the connections and case can corrode.
High as possible under the scuttle in the passenger side tunnel with a foot guard over the connections would be good.
Not below the wiper spindle openings which could leak.
Wet floors can form condensate on the bottom of the scuttle and ecm, so there should be good drainage in the floor.
FYI, oems typically place the 90s ecms behind kick panels, in the dash, and for miatas, behind the passenger seat, nearly always vertical with the connection on the bottom.
If you can keep it dry, it shouldn't matter so much, so flat on top of the tunnel and covered, with no possibility of water running under it works too.
Dielectric grease is about $10 for a tube about 5 inches long, so not cheap but a little goes a long way. It must be non-conductive grease so you are not shorting out between pins. Every connector should be greased. It will make the car more reliable in the long run.
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
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360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12