@Gomickeygo you didn't say what you're putting them on. I'm running Doug's D567 round port headers on my '69 GTO. They fit "fair". (All headers suck, some just suck less).
I had 3 issues on my '69:
Had to move the brake distribution block to the top of the driver side frame rail from the inside of the frame rail. Otherwise it would have been way too close to a header tube, cooking both the fluid and the valving/wiring in the block.
Had to "re-bend"/re-engineer the steering column to back-drive transmission linkage in order for it to continue to work properly.
Had to swap the lower control arm bolt on the passenger side "end for end" so that instead of the bolt end and nut, the bolt head is now next to the header tube, otherwise the end of the bolt would have been rubbing against the tube and would probably have eventually worn a hole into it. This means that when I eventually have to service the lower control arm on the passenger side, I will have to loosen that header enough to move it out of the way so I can get the bolt out.
I've seen in the past that the much talked about "supply chain issues" making new headers hard to find at retail. I don't know if that's still the case.
Another recommendation I'd make, if you choose to to with headers, is to NOT buy the coated versions. Instead buy them in bare steel, then send them off to Jet Hot coatings and have them apply their high temp ceramic to them. Their coatings are much better than what you get from the header manufacturers. I have their "level up" extreme temp coating on mine in a shade of blue that goes well with the silver blue metallic engine color.