It should be easy to get the shocks, and install them -- I did that, and added a 22mm IPD rear anti-sway bar at the same time (also installed the 25mm front bar, too, of course). You might take note that the front shocks are as easy, if not easier, than the rears, because unlike the 240's, the 164 still has the old double wishbone suspension instead of the MacPherson struts, where you need spring compressors and all sorts of work.
The seats, front and rear, can be swapped with any 240.
The rear brakes are essentially early 240 -- check with your local parts guy.

Which brings us to the "front" of the car ... unfortunately, the front brakes are rather unique -- hard to find calipers, because they were only applicable for a few years, and are in short supply; rotors may be confusing to many parts stores, too.

I believe the steering rack is unique, too (was once considering buying another '75 164, with a bad rack, and I tried pricing a replacement to figure whether it's worth buying ... I couldn't locate a replacement rack). My "theory" is that because the '75 164 has a unique track*, that year alone needed a different rack, and it was only in production one year -- so again, they're in very short supply -- at least, that's my theory.
[ * Pre'75 164's had a suspension and wheel offset that was pretty much consistent throughout the run; but in '75, for its last year, Volvo gave the 164 the same track and suspension geometry (and calling for a new wheel offset) that matched the 240 series. So I suspect that is why the steering rack is so unique.]

And you can't find a replacement radiator anywhere. The later 240's radiator doesn't fit, and neither do earlier 164's or 140's.

And I'm sure I haven't discovered all the things that are unique. I'm in the process, though. :-).