Good post Mike...I just want to reiterate that the method I outlined earlier is a fix for the situation we have today and if the repairs had been done correctly originally we wouldn't be having this thread going on now...Mike is right, what else may be buried underneath the primer and usually if there's something obviously wrong, there's something else wrong that isn't obvious.
I've used the 1/8th welding rod to get proper gaps on pre 1940's cars only so far, the gaps that are present in many of these 70 plus year old cars may have existed from the factory or over time they reared their ugly head...that being said...it may help the OP to do a repair that perhaps isn't 100% and wouldn't recommend this type of method for say a #'s matching LS6 Chevelle (that's why I asked if the car was extra rare). The proper fix would be to remove the quarters, install them using the deck lid to achieve the gaps wanted, an alternative to that would be to use Tech 69's approach. We now have 3 methods to solve a situation that was created...the choice is up to the OP and I would welcome any other recommendations. It's the OP that has asked for help and advice and I hope the solutions offered help in his endeavour.