72 GMC heater core replacement

[Glennerd]

I am about to replace my heater core and I would really like some tips as to what may save time and what needs to come off for access. My Chilton is in the truck, truck is not here and no internet where the truck is. I have gobs of crap squishing out at the firewall, covered by what seems to be some type of undercoating. Right away, the flag is up. I am replacing the driver's fender. Someone had told me that the pass. fender had to come off? If so, not a huge deal, but I can't look at it right now. Any help would be great. Really only need a quick run through, unless there is something I don't want to know. Ha ha.