Wow you’re making great progress! Doesn’t look like you can get that window track as an aftermarket item. Probably going to have to source that used. As for the door handle, yes there should be a spring in there. Here’s a link to classic parts for a new pair
https://www.classicparts.com/1952-59...ctinfo/18-107/
One other thing to be aware of as you continue to add weight to the front. Your lower a-arms should be parallel with the ground at final ride height. I know you didn’t install the front end, but something to consider. May have to get lighter springs.
Otherwise keep up the great work. Those latches look awesome. I’ll be following!