Early Abody Doug’s 450 header install BEFORE engine going in

Doing a /6 to 340 swap in a 65 Valiant, have everything ready to go in the car.I know there are a lot of threads and great info about how to install these headers with the engine in the car. The engine is not in the car, steering columns is out, torsion bars are slid back. Wondering if anyone has or would it be easier to put the headers together as far as the slip tubes and flanges etc. lay them in the engine bay set the motor down between them then bolt them up. Was worried about putting them together and it not working then having to deal with the sealer and the used (at that point) flange gaskets if the install like this won’t work.