Trunk gap problem after new 1/4's
Whatever you have to tell yourself.
The panel fits on these cars were NEVER perfect, not even when new. Body measurements could vary by as much as a 1/4” from the factory. What fits one car might need significant adjustment on the next, and the gaps sure as hell were not all the same. And no reasonable person should ever expect any replacement panel to fit without bodywork, that's not real life. And that's not just Mopars, that's universal. If you need your replacement panel to fit perfectly out of the box you're destined for failure.
The door gap widening an 1/8” requires something moving. The quarter panel just has a flat fold that gets welded directly over the door pillar upright. So, in order for the door gap to change, the door pillar upright had to move. And subframe connectors won't stop the door pillar upright from flexing back an 1/8", or even a 1/4". Not what subframe connectors do. Maybe the panel pulled the door pillar upright, but that happened as part of the installation. Since you said you had the doors in place when you did the quarters, I assume you had the gap right before you started welding. Which means your welding process pulled the panel. Otherwise, the time to address that 1/8" difference was before you welded the quarters on.
If you want your car to have perfect gaps after replacing a substantial amount of structural sheet metal, it will require a ton of work and adjustment to get it right. Even perfect panels won't change that.
Whatever you have to tell yourself.
The panel fits on these cars were NEVER perfect, not even when new. Body measurements could vary by as much as a 1/4” from the factory. What fits one car might need significant adjustment on the next, and the gaps sure as hell were not all the same. And no reasonable person should ever expect any replacement panel to fit without bodywork, that's not real life. And that's not just Mopars, that's universal. If you need your replacement panel to fit perfectly out of the box you're destined for failure.
The door gap widening an 1/8” requires something moving. The quarter panel just has a flat fold that gets welded directly over the door pillar upright. So, in order for the door gap to change, the door pillar upright had to move. And subframe connectors won't stop the door pillar upright from flexing back an 1/8", or even a 1/4". Not what subframe connectors do. Maybe the panel pulled the door pillar upright, but that happened as part of the installation. Since you said you had the doors in place when you did the quarters, I assume you had the gap right before you started welding. Which means your welding process pulled the panel. Otherwise, the time to address that 1/8" difference was before you welded the quarters on.
If you want your car to have perfect gaps after replacing a substantial amount of structural sheet metal, it will require a ton of work and adjustment to get it right. Even perfect panels won't change that.