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

Thanks, good info. I'll be doing this on the ground. This has what I think is the original 6 cylinder radiator, and I'll replace it with a Cold Case or something. Will having the radiator out of the way give the clearance I'd need? Are you recommending pulling the trans first and then the engine, or pulling them as one piece?

I've read that with the transmission attached it really takes two people to get it over the core support, although jacking up the back end can help with that, and even airing down the front tires. Using a ratchet strap to support the trans while removing the engine and then supporting it with a floor jack when it's going back in is another suggestion I saw in an article here. With the transmission attached you need a leveler (which I have), but you lose a fair amount of lift height needed to clear the core support. I suppose a gantry would be ideal, but I don't have one. A four post lift in the garage would be really nice.

If I do pull the transmission, I'd also have to disconnect the 1&2 and 3&4 shift linkages, pull the drive shaft, and probably remove the cross member. If I have to, well, then I have to. But I was kinda hoping to avoid that :)
Have the radiator out is helpful. It can be done with or without the trans attached to the motor. With it attached, you really need one of the engine adjusters like this
https://www.harborfreight.com/3-4-quarter-ton-capacity-heavy-duty-load-leveler-67441.html
I find it easier to pull the trans first.