Sorry for the late reply. As a 70s bike that one is technologically way behind Miyata models from the 80s. The main functional difference is weight. That Liberty model will have high-tensile steel for its frame, fork and stays. By the mid 80s all but the very cheapest models from companies like Miyata were made from alloys that combined strength with lower weight. Also that bike will likely have steel wheels; again during the 80s alloy rims became mandatory on all but the cheapest road bikes. Components, too, would be better on an 80s bike, but that probably doesn't matter to you.
You are right about the bike being from (probably) 1978, according to C&Ver T-Mar's detective work on Miyata serial numbers. They apparently began dating frames with a letter prefix with 1972 as 'A.' So G=1978.
As a frame for a fixed gear conversion that bike is probably very solid, but heavy. Be sure and check for rust, especially in the seat tube and headtube. (ie pull the seatpost and stem to check before buying)
$80 sounds a bit high for it, but that's probably what I would ask if I were selling, too.
