Removal of heater box without removing the inner fender on a 67-72 C10?

['68OrangeSunshine]

I paid a Radiator guy to solder my heater core end nozzles back on. They had gotten loose from wrestling the hoses off. He did a very neat job.
Then I was installing the heater core in the fiberglass housing. I used a power drill to shoot the screws on. I dropped one of the original screws -- couldn't find it -- so I got a 1'' drywall screw and shot that in. Put it all back together.
And it leaked!?
The drywall screw was sharp and had pierced the brass tank end.
I had to start all over again. Luckily, I found another tank I 'd squirreled away in the depths of White Fang and forgotten.