You don't say what year yours is. That makes a big difference.
There are some rally gauge sets that will work that will bolt right
in with a lot less work than you describe, if you can find them.
I think the later system started in 73 and it sort of blends into the
240 dash system. 67-72 is different, but I think there are rally gauges
for both kinds.
Matt Bartmann at Forcefield.com is more familiar with them than I am.
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George Downs Bartlesville, Oklahoma