Header install?

I’m throwing the idea of doing a Gen 3 swap in my 71 Dart. Question is about TTI header install. Looks like the flange that bolts to the heads is hard to access once in the car. At what point of the swap are you guys installing the headers? While installing the engine with it on the hoist? Or can you access the bolts from under the car?
I installed my headers after the engine was in the car. Granted I have an HDK crossmember, so there is less "stuff" in the way. A couple bolts on the passenger side were challenging to reach even with the HDK. They may be impossible with a stock K. @JBrian could provide some good insight on this.