on the 78 it should be no big issue, as the a/c compressor is on the other side. you may not even have to lift the motor. that said, if you do have to unhook it, I'd just pop it loose at the firewall. I have a 78 parts car, so I can take a pic for you if you like... and make suggestions for you.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!