You are correct, they are not easy to get and expensive. My buddy just went through this on his 67 cuda. Has the 5.7L out of a 03 ram, took him forever to locate a new apps ...
He is currently finishing his 72 scamp. Has the drivetrain out of a '12 srt challenger. 6.4 and the 6spd, he's using the challengers throttle pedal on a modified mounting bracket.