"Will the pressure readings be accurate, given that I will be using the gauges intended for an R12 system?"

Yes, the pressure readings will be as accurate as before. But the conversion to temperature will be different.
http://www.csgnetwork.com/r134apresstempconv.html

"What do I buy to have a hose that will accept (on one end)an r134a can of refrigerant and (on the other end) will attach to the middle of the R12 gauge set (or the existing R12 hose)?"

Maybe http://www.ebay.com/bhp/r12-to-r134-adapter

"Where exactly is the high side port."

The condenser (in front of radiator) is on the high side, so look for a port on the line leading from it into the passenger compartment. That port is used to check high side pressures and not to add refrigerant.

Note: If you want to add refrigerant, add it to the low side. There's probably a hard to access port on the compressor for that.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.