Guess that I was distracted and missed that. You did not say if it had A/C. I would think that it's similar to my '69. Remove the radio/heater face plate, two screws that hold the radio. In back there should be a support bracket, antenna, power, and speaker wire. Should drop out. Sorry if the '68 is different than my '69. If it has A/C, there are 2 studs with nuts that hold the louvered air distribution under the dash, it has to come out. It's difficult to snake the radio down between the dash and A/C box itself, but it can be done.
