So much has changed in the last 45 years with the accuracy and quality of injection moulding tools. If the machined angles of the cavities inside the tool don't meet perfectly then hot liquid plastic under pressures ranging from 40 to 90 tons is going to find a way, in a just a few seconds, to find the weakest point in the tool and 'flash' out.
This is actually one of Revell's worst releases. It is wrong on many levels. Several folks tried to work with it many years ago and the Zee Toys monstrosity isn't much better. This model and the slot version of it are why I got into 3D scanning and printing.
You've done a nice job with it thus far though, and the Boyz should be proud.
I have been trying to teach myself how to make my own plastic injection molds for the past few years. I feel confident enough to make Palmer Plastics style kits but I have been thinking they are simply not going to ever be accepted compared to todays current high quality kits.
Someday I am going to be able to actually make sliding side model kit molds but to be honest I would be very happy if my stuff even resembles the real cars. The process is not impossible, it can just fail for the smallest of reasons.
I have a 28 ton Arburg in the shop and a couple of CNC machines. All the stuff I have is really low price and antique. I am currently hooking up a Hansvedt EDM sinker to help smooth things out. It is from 1984 so I hope it works OK.
The goal is to make stuff never before available. Someday......