Respect for doing all that with the engine still inside the car.
I did a similar job on the engine but pulled it out for painting and repairing threaded holes for the exhaust manifold bolts.
Mine is a 440 with headers though, so even more limited space wise.
Which heads did you end up using, the original ones or the spare set?
I had the old head out to pull an exhaust stud, so I put a bit of yellow paint in the flaw under the seat. I then lapped in the valve just to see how much seat there actually was.....not much, I'm surprised that it sealed.