Welcome! I love those 87 442s.
As noted, there's a lot of info missing on the "how to" part of it in the video if someone wanted to emulate your efforts. You certainly are committed to it and you have absolutely NO idea how much it pained me to see you cut into that pristine door like that. I'm a big fan of rock-stock purist when it comes to my own cars, and for me, that video does not apply. I'm really glad you're enjoying the car, though, however you wish to enjoy it.
Maybe do vid cap pics on some of the stuff so you can have video AND pics/text for searchability? Just a suggestion. I know it's a PITA to do it either way, but it's just one idea.
Modifications to your car are yours, and you own them whether they come out great or they suck. From my perspective, you did a good job of replacing and installing the door handle with one you feel works and looks the best. I'm still not sure how I like it yet. I like your idea, but the handle itself is rounded on the ends, where hardly anything on a G-body is rounded.

That's why the stock rectangle lift handles seem right at home on a square-ish G-body. I view them as a refrigerator box that wants to be aerodynamic.
As far as suggestions as what to do next, that's a loaded question. What is your long-term goal with the car? Drivetrain? Stock-ish or are you going to toss in an LS swap? Lowering suspension or leaving it alone? Tubbing? Racing? A zillion ways to answer the question. That's what I would do, set a goal for you and the car. What is your dream 87 442? Answer that and go from there.
Regardless of which way you go, you said it was a NY car. Rust in the door's bottoms are the least of your worries as you have a T-top. As everyone with a T-top G-body knows, there's a good chance that there is some rust on the sub-structure up under the weatherstripping and top plates. So THAT is where I would go next. Because if there's a substantial amount of rust, you're in for a challenge. If not, you're golden and dodged a huge bullet. Or, I should say, cannonball. You said you already replaced the floor pans, right? And the reason they were crusted out? Could be the road salt spray, true, but T-top leaks also were a huge issue as well. Pull the A-pillar inside trim. Easy peasy to do, just a few screws and inspect well behind there and see what you got as a precursor to pulling the rest of the trim. I've seen where water actually travels down the A-pillars on the inside and rusts out from in there and even caused windshield leaks that were hidden by the trim. Potentially a horrible mess underneath while looking perfectly normal from the outside. Don't want to scare you, but be ready. And check those rear frame rails right behind the rear axle. That's another area of concern on these cars, especially if they lived in the north.