Installing headers -- While engine pulled or after re-installed?

I installed Doug's (D453) headers into my 70 Duster without any issues. Headers mandate going to mini starter. Only issue was after I changed the torsion bars to 1.03 I had to clearance both sides. I simply raised each side and slid them in from underneath. You don't have power steering so piece of cake. I had to take the car to a muffler shop to have them fabricate dual exhaust out the back. I put on IE style chrome tips while I was at it and got the mufflers I wanted. Very good power without a lot of obnoxious noise too!