I too had this trouble worked out cheaper and easier in the long run just to slap in a set of std mfi clocks as by the time it takes to find someone who can do all the relative working out to tell you what size resistor to put in the circuit to get it to work anywhere half way decent you could have pissed the job off and be onto the next headache ................................. besides if you go looking hard enough you can pick a set of clocks up for a tenner or less than if you know the right ppl.
Oh then you got the next probem of which clocks will work the best with your fuel sender unit as 90spec clocks on a 88 harness just dont really work below 1/2 a tank