If the 78 has an electric oil pressure gauge, and all the same idiot lights / gauges are your pod -- It'll be a plug n' play setup, the 87 will also be a plug an play setup.
Check the Tach info thats found in the Interior part of the Garage section of 73-87.com (woah, that was a bit of a mouthful,
) It'll have all the info you need, incase I am wrong (I'm fairly sure I'm right though!)