On how old hardware can modern engines be run, and how strong are they? For example, could modern Stockfish theoretically, with the hardware at the time, be entered into the 1974 World Computer Chess Championship?  If so, would it be grandmaster level?
could modern Stockfish theoretically, with the hardware at the time,
  be entered into the 1974 World Computer Chess Championship?


No. Modern Stockfish is not compatible with 1974 computer architecture.

Modern engines which can run on 64-bit Linux should be able to run on an early 90's Digital Equipment Corporation 64 bit Alpha PC running Unix with a suitable recompile and perhaps tweak of the code.