Was/is the 757 classed as a 'heavy' for ATC purposes?...I recall some comments about powerful vortices from it, though its weight and size were in the 707/DC-8 bracket
Yes.
From the UK AIP:
"All Boeing 757s are classified as Upper Medium for wake turbulence separation application, irrespective of weight. This is due to an unusually high core vortex speed as generated by the B757 wing."