I have a 67 cutlass c/s conv with a 71 455 got a set of used headers the motor is out of the car should i put the headers on the motor before install or just set them in loose

 

You can set em in loose and while lowering engine dont crush em. Make sure they are good to bolt on before bolting engine to mounts. I dont think its possible to install em and then lower engine into place.

 

I have a 67 442 conv with a 425....hooker headers fit well. I dont know about other brands but some tweaking of tubes may be necessary or corner of upper control arm on passenger side. Your brake junction block will need to be moved from the side of the frame to the top.

 

I agree. Put the headers in the car and lower the engine in on top of them.