I think book hours to do it are probably close to 8 hours, and it would probably take them almost that much time if they haven't done them frequently.
I have done one on my 81 Fairmont and 79 Zephyr, and 89 Cougar, and it is no fun at all. Dash has to come 99.9% out, with it being 18 years old, dash stuff is going to break.
I would do two sets of HGs on a 3.8 before another heater core, but if it needed done, I would do it. It is time consuming and tough to not break stuff.
97 Mark VIII LSC/Silver/Gray leather
97 Cougar 4.6 Sport/White Opalescent Tricoat Metallic /Gray leather down but not out?
89 Cougar LS 3.8/Oxford White/Gray velour/HG need replaced AGAIN
00 Grand Marquis LS/Vibrant White/Gray velour
63 Falcon 302/black and rust/shed isn't moving
82 F-150/Oxford White/dumb VV carb doesn't pass emissions