[Steve@OldSub.com]

Its not a '77, the long trailing arm rear suspension was only used through 1972. I see front coil springs which mean '63 or later. The power steering box appears to be a factory mounting, which does narrow the frame to later in the '63 to '72 range but I'm not sure how late.

I can't see the front brakes. If they are disk and we can assume this is an unmodified frame, its a '71 to '72.

So not definitive, but that is what I see.