If you want to strengthen a join, apply a fat line of regular superglue along the inner side of the join, then cover with baking soda. Only do this where you won't want to remove any, as it sets up almost like stone. I do this quite a bit, since I only build customs myself.
You're brave doing a custom for your fourth model. Good start with the new trunk area. A candle!?! More bravery! What kind of plastic did you use? I try to make sure body parts are very well attached and supported I often use super glue plus baking soda for an easy reinforcement on the back side where it won't show. Helps to avoid problems when I'm cutting, shaping and so forth.
Looks good. I tried to help by finding photos of the C1, particularly rear and side views. I've attached what I found. The-Blueprints.com does have a four-way of the C4. I'm wondering about the rear fenders: the C1 appears to have blunter-ending fenders than the C2. Perhaps similar to an early Vette.