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Should I remove R12 (390 ohm)? Remove R42(1K ohm)?

The new 450 ohm resistor should be 1/2 watt, right?

Yes,Yes and can be a smaller wattage, it is only firing the LED inside the opto.

The instructions you refer to use about 1100 ohms, 450 seems to be more reliable. There has been much more development over on the Extra site.

XG1 to XG2 is the original ground path from way back, no longer used.
Here are two pics from two years ago on one board, this is heavily modified. It now is running the MS2 processor.
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[Linked Image]

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.