Zack, here's the deal......
As Frank mentioned, "anything is possible" but...I've owned the actual 1:1 cars and the models and I can tell you that ONLY the chassis and some trim are truly interchangeable. Years ago I posted a fair critique of Sunstar's 1/18 1959 Dodge. There's MUCH MORE wrong with it than just that "droopy" front clip. A 1/18 1957 or '58 Plymouth won't help you either. Again, no body panels were shared. As Frank pointed out (and I agree).. your BEST BET is to RE-WORK Sunstar's 1959 Dodge and if needed "kit-bash" parts from ANY source.As you may know... I've done MANY indepth mods... a "nicer" 1/18 1959 Dodge is possible but it would be A TON of work!Good Luck!