Carpet install 68 fastback

Do not cut the trimmed edge off the front carpet, it is there for a reason.
As stated by others, you trim the front edge of the front carpet that is by the firewall, and sides as needed
I checked mine, the front carpet lays on top of the rear carpet, overlaying by approx 1.5"-2".
Based on your pic, rough idea would be the finished edge of the front carpet lays right in front of the center console mounting bracket.
If its molded to fit into floor pans, line it up based on that and trim up front.