You have to be INSANE to have long-tube headers on a big-block A-body Mopar!

I don't think you need to take the entire drive train out, although that is the reverse of assembly - many people have put the whole engine/transmission/headers/K-frame in all at once. You can put it all on a floor dolly, and lower the car on top of it with a cherry-picker.

I don't have a lift either, so I put the drivetrain in from the top, then jacked the car up, put the headers in, balanced the K-frame on a floor jack and lifted it into place.

It was somewhat easier for me to do it in that order since I have an AR motor plate and homemade frame mounts, so I didn't have to support the engine across the fenders or with the cherry-picker... in fact the engine had to go in from the top since the front plate is wider than the frame rails and I'd already put the water pump on :)

Anyway there are lots of approaches, all of them stretching our sanity... Good luck!