He is right (of course), I used urethane on my 70 Challenger, had a guy come to the house and install it (twice because I broke the 1st one the same day it was installed....
). I used butyl tape on my 73 Cuda and most glass companies will not use it because the US DOT outlawed its use in passenger cars in general. That said, if you search around and talk to the glass companies and tell them it is a classic car they may use butyl tape for originality.
Now, I am sure people will jump on here and state that butyl is "not safe" and you could "fly out the window" but it was used for many years and I doubt you are using the car as a daily commuter vehicle, I would not hesitate to use it again.