I thought you'd mentioned that you had the tacho working it was just reading wrong?
If you have the tacho reading (albeit wrong) then you very well might not need a booster...
Not being familiar with 180SX's I'm not sure if it is possible to take the dash apart and 'calibrate' it to work with the 6 cyl tacho output from the 1JZ....
If it isn't possible to calibrate the dash then one solution might be to use something like a Jaycar speedo corrector to correct the 1JZ tacho signal... and you might need a tacho booster after the speedo corrector depending on what signal your dash works off...
e.g.
http://www.jaycar.com.au/productView...o&form=KEYWORD
Cheers
Wilbo
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