Schumacher headers 440 in 71 Duster

Should I be bolting on the headers first and installing engine that way, or putting on the headers after engine is in? I have a manual box.

Watched roadkill garage latest on the Dart they bought with the 440. They re did a whole bunch of stuff and added Schumacher headers.

They had trouble mounting while engine was in car but that was with a factory PS box.