Looking for the most detailed model of a diesel from the pre-1979 Conrail period?
That is not the easiest question to answer at all.
My answer is going to be "it depends based upon the specific model in question".
If you truly want the most road-specific detail, Overland Models brass might be the way to go, as they have generally done a great job especially with diesels from that particular era...but then many folks will complain about the price and say they want plastic only. Now I myself have had the occasional QA/QC issue, but generally speaking, with Overland Models you are going to get exactly what you pay for.
Most of the prototype diesel models in question are far from being the newest/latest diesel in real life and as such have escaped the detail upgrades that other models have received in recent years.
Kato anything--I'd pretty much forget it because they were generic models with roughly generic features and few road specific details. Upgradeable, sure, but you must invest the time, research and modeling skills to do it.
The Atlas GP40-2 and C-420 are the best detailed models they've ever offered (not coincidentally also the most recent non-Trainman releases.) Anything else they have at this point is somewhat dated regarding detailing, and will not meet the "best detailed" criteria, period.
Bowser has done a terrific job of upgrading the old Stewart C-628 and C-630 to today's detailing standards, and has a brand new from the ground up C-636 coming. These all are terrific models, though C628/C630 are still not 100% correct for all roadname variations because the Alco prototypes changed so very often--it would be virtually impossible to produce one plastic big Alco Century 628 or 630 model that would be dead on 100% accurate for more than one or two railroads. They've done a great job with the PRR/PC/Reading/Lehigh Valley/Conrail big Alco's--even offering the correct Conrail patch paint jobs for correct individual units based upon the prototype photos. The Bowser models are now loaded with separately applied cast brass details and still available at a "reasonable" price, yet will compare very favorably against the brass imports that have been done in the past.
For late F units surviving into the 1970's, the Athearn Genesis models have been the way to go. Buy the Ex-Rio Grande lettered with Penn Central lettering F-Units. They were awesome! Extremely well modeled if accuracy and fidelity to prototype is the primary concern!
Also the Walthers Proto 2000 Alco RS-27 in Penn Central and Conrail. Excellent, exquisite model! Great handrails can actually be handled without breaking and stay looking good, and as accurate a diesel model as I've ever seen--and they lasted well into the '70's in real life.
I do not believe you are going to find an SD40-2 that is in the same detailing league as any of the specific units I've mentioned above...Excepting recently tooled Canadian releases.