Needing a return line on a carb fuel system isn't based on how much pressure you have; it's a means to keep the fuel cool and prevent vapor lock by having a constant flow of fuel from the tank to the carb and back instead of dead-heading at the carb and potentially heating up and causing vapor issues.
I watched a YouTube video recently about tuning carbs with modern gas and the guy (Thunderhead289) explained that without a working fuel pressure regulator and accurate fuel pressure gauge your carb tune will change pretty drastically as the engine goes from cold to warm to hot. And also adding a fuel return to any carb fuel system will make it run more consistently. Now that doesn't mean that's the cause of your issues but it may be one of those things where it's just bad enough that it's making things run worse than they should be.
Like others have said, overall I think you should try your best just taking things nice and slow. You mentioned being more of a body guy and not being as into the mechanic stuff, I'm the opposite. And for that reason if I ever decided to take on a bodywork-related project I would spend TONS of time reading, watching videos, asking questions etc. before I start doing anything because I don't have the money or patience to ruin my car and have to start all over again. Same goes with engine stuff. A person can do one seemingly insignificant thing wrong simply from an innocent lack of knowledge and his entire engine is destroyed. It takes a lot of time, patience and careful detective work to really do a proper autopsy on a broken engine and if you skip a step (like you already did by changing the oil twice and throwing out the old filters without cutting them open to look) it just puts you 3 steps back.
It might be painful and frustrating to slowly pull the engine apart bit-by-bit as opposed to just pulling it, selling it and dropping in a 383 but I guarantee you will see things that will make you go "Holy Sh** THAT'S what happened?!" and you will only gain knowledge that will help you in working on old Mopar V8s in the future.
