Using the original as a template reminded me just how strong the drawing chops were of the average cartoonist/illustrator back in the day. Mr. Cross probably whipped out the drawing in a couple hours, but his obvious training and study of anatomy and the human form are evident.
There were things you wouldn't dare change. Stan's composition is perfect, the body language masterful, and the expressions hit exactly the right note. Thank you sir, you've already done the hard work.
A couple things seemed easy and obvious to change; adding those futuristic buildings and flying cars that Darren wanted, and making the second character female. Gender balance and greater embarrassment and humiliation for the other character - win/win!
But that building they were hanging off just didn't want to become futuristic without completely altering its style and making it unrecognizable. I wasn't satisfied, but plowed onto the pencils hoping it would either stop bugging me (never happens) or a solution would emerge (only happens when I'm very, very lucky).
I got lucky. Half-way through the pencils I decided to leave the building in it's original design, only in the process of demolition, not construction.