@mr2m1 I’m a year and a half into making some 60 Series products and I’ve run into this same exact thing. The US-spec 60 that “was the same” from 81-87 actually had a whole lot of very small changes year to year. I’ve had the same thing happen where I make a product for my truck that I’m happy with, go to install it in another 60 and it doesn’t work, so I scratch my head.
Once you figure out the difference issues I’d gladly participate in a survey: send you photo of the area in question along with the month/year on the build plate. First you gotta figure out what the differences are - the wire bundle in back shortening your depth is one. Maybe a bend of the sheet metal dash is changed by a few degrees or the mounting holes are off. Happy to send a photo of mine (1982) if it helps at this stage.