If you already have engine installed (which I assume you do), maybe someone else here can give you some advice as I have only done it with engine out of vehicle.
If the engine is installed, either dropping the K-frame/engine assembly or lifting the engine up in the engine bay. If lifting the engine, it doesn't need to be completely removed, but will need to be lifted to install the headers (losely), then drop back down.
So there are two options to install headers with the engine installed... for a 440 anyway.
Tom