79 dash gauge swap for 85?

[anteken]

Greetings,

this is may first post and I have what I hope is a simple question. My 79 has the strip speedo and I want to swap for the round gauges with tach. The used ones I have seen show 78-80 and 81-87. Is there any diff between them if you have the backing with the gauges already mounted on it? will the 79 cover fit correctly over the round gauges? Anyone who knows...please help before I buy something that might not work.

Thanks much!