I ran into the same problem with my car, I had no mounting plates on the frame to go with a 76 400. If your 77 400 block has all 5 motor mount holes on each side threaded like mine does than Ames sells what you need. I just bought the same thing,
$155.00 to the door, it comes with both frame plates, the rear support brackets, and some awesome looking motor mounts. The guy at Ames knows what he is talking about, I looked everywhere for something that would work and nobody but Ames had it, $155.00 ain't cheap but the stuff is real good quality.