That's a tough one--I'd start by noting the vastly different elements of mix style than would be the case in something modern. There's a severe lack of modern brittle high end. I'd think about what tools they used then and try to emulate it digitally. Off the top of my head I see using low pass, distortion/saturation on majority of material. For the harmonic instruments I'd see going for a room sim sort of verb, to severe compression, to a subtle/vintage/saturation/distortion, in that order. Maybe another verb (spring) after the fact. They have free Bricasti IRs online, I'd go to that folder first. For those 70's drums, some compressor that's doing that "steely dan" sort of thing, severe threshold, severely panned, fast attack/slow release, something like that. Generally avoid modern mixing conventions and try to A/B and match what you do to that material. There's always downloading the immense amount of vintage plugins to be had as well, but probably without the above stuff, it won't be enough.